How to make a boomerang

by Jearl Walker

A BOOMERANG is surely one of the oddest devices ever to serve as a weapon or a plaything. It was apparently invented by accident, notably in Australia by the native people but also independently in many other places. If you throw an ordinary stick, it falls to the ground not far away, but a boomerang can travel as much as 200 meters (round trip) and can be aimed so skillfully by an expert thrower that game or an enemy can be hit. The boomerang probably originated as a weapon designed for accurate straight flight, but most people find the returning boomerang, which was mainly just a plaything for the Australians, more interesting. Ironically the straight-flying boomerang is probably more complicated aerodynamically than the returning boomerang. As ancient as both devices are, an amateur investigator can still do a great deal to advance the understanding of their flight.

Although good returning boomerangs are occasionally available in sporting-goods stores, most commercial returning boomerangs are mass-produced and fly poorly. Indeed, many of them do not even return. The plastic boomerang made by Wham-O is one of the best types available in toy stores. Many other types, all of them excellent, are available from Ruhe-Rangs, Box 7324, Benjamin Franklin Station, Washington, D.C. 20044, thanks to Benjamin Ruhe, a boomerang enthusiast formerly with the Smithsonian Institution. During his service there Ruhe helped to organize the Annual Smithsonian Open Boomerang Tournament. The tournament, held in late spring or early summer, is great fun for the 100 or so contestants who enter. This year it is scheduled for June 9 on the mall in Washington.

If you would like to experiment with boomerangs, you need to be able to construct your own. Only then can you make the variations necessary to determine what factors influence a boomerang's flight. The best material from which to cut the basic stock of a boomerang is Baltic birch in a marine or aircraft plywood between 1/4 and 3/8 inch thick, with five or more laminations. This type of plywood is resistant to wear and water; it is also dense, which makes for a boomerang that is heavy for its size.

Herb Smith's "Gem" design for a boomerang
Cut a cardboard pattern for a boomerang of whatever shape you want. An example is the returning boomerang designed by Herb Smith and shown in the illustration at the left. (If you are left-handed, you will have to make a left-handed boomerang, which is the mirror image of the one drawn.) Place the pattern on the plywood and mark off the outline of the boomerang in pencil.

With a coping saw or a handsaw cut out the boomerang blank. The edges and the top must now be shaped in the general form shown in the illustration. (Little is done to the bottom other than putting a slope on what will be the leading edge.) The leading edge must be blunt, whereas the trailing edge must be sharper, with the top surface sloping down to meet the unaltered, flat bottom surface. The arms must have the cross-sectional shape of a classic airfoil, because they must provide lift much as the classic airfoil does.

Clamp the blank in a vise and cut and shape the edges and the top with a rasp that has a curved surface. Smooth out the grooves left by the rasp and finish shaping the wood by rubbing the surfaces with a piece of coarse sandpaper wrapped around a piece of soft wood. Before you finish off the surface with a finer sandpaper you should make a test flight with the boomerang so that you can tune it by more shaping with the rasp or with coarse sandpaper. Tuning the boomerang means that you throw it, cut or sand it more and then throw it again until it flies the way you want it to.

A boomerang cannot be thrown well in a strong wind. If there is a light wind, face toward it, turn 45 degrees to the right and throw the boomerang in that direction. Hold the boomerang vertically by the tip of one of the arms (which one usually does not matter) with the flat side away from you. Reach behind your head with the boomerang and then throw it toward the horizon, snapping it forward when your arm is fully stretched forward. Do not try to throw hard at first. It is the snap that counts, not the strength of the throw. The boomerang stays up in the air because of the spin the snap imparts to it.

The proper orientation of the boomerang (the plane in which it will be spinning) will vary according to the wind conditions and the type of boomerang. To achieve a good flight you might have to throw the boomerang with it plane nearly vertical. Under other circumstances you will have to shift the plane (tilting the top of the boomerang away from you) by as much as 45 degrees. The greater the tilt of the plane in which the boomerang spins is, the more upward lift the boomerang will initially have. If you give it too much lift, it climbs too fast and then plummets to the ground with such force that it may break.

Preparing to throw a boomerang
In a good flight a returning boomerang travels horizontally around an imaginary sphere. On returning it probably will hover or even loop a bit before it drops to the ground at your feet. If you are lucky, it might make one or two additional circles (smaller than the first circle) before it falls. Although you launch the boomerang with its spin plane almost vertical, it probably will return with the plane nearly horizontal. I shall explain below why the plane must turn over in this way if the boomerang is to complete the trip.

If your boomerang consistently lands to your right in a light wind, try throwing it a bit more to the left of the wind. Similarly, if it is landing to your left, try throwing it more to the right of the wind. If it lands behind you, try throwing it with less force. If that does not work, throw it somewhat above the horizon with the spin plane tilted less from the vertical. If the day is windless and you are not getting a full return, tilt the spin plane farther from the vertical in order to gain more lift during the flight.

Be careful not to injure people or damage things with your boomerang. It can be quite a weapon. Throw it only in a large, open space. If people are present, make sure they know what you are doing so that they can be prepared to dodge.

The successful tuning of a boomerang involves both experience and luck. In general if you make the top surface more curved, the boomerang will have more lift, which means that it will return in a tighter circle. Flattening the top surface or curving the bottom surface will give the boomerang less lift because the cross-sectional shape of the arms is then less like a classic airfoil. If the spin decreases too fast, so that the boomerang falls to the ground in mid-flight, the reason may be that excessive air drag on the arms is robbing them of their spin. Some surface roughening might be beneficial to the flight, but any large grooves left by the rasp will surely create additional air drag that will shorten the flight time.

Instead of shaping the arms carefully you might prefer to twist them so that during a flight the leading edge on each arm is tilted to deflect the passing air to the right, giving the boomerang a lift to the left. This type of lift is easy to visual window of a moving car and turn it through various angles, you can feel the lift. To twist a boomerang heat it gradually in an oven at 400 degrees Fahrenheit and then (with gloves on, of course) carefully twist the arms until the wood is cool. If you twist too much, heat the boomerang again and twist the arms back a bit.

A typical flight path
Once you have a properly flying boomerang you might want to finish it-off with a cellulose covering and some decorative designs. Smith's excellent booklet, which is listed in the bibliography for this issue [below], explains how to do this kind of finishing and also gives a number of boomerang designs. If your boomerang breaks, do not throw away the pieces. Glue them together with epoxy, clamp them until they dry and then file and sand the surface back into the desired shape. Although the boomerang will not be as strong as it was before, its flight path might be altered in an interesting way by the small change in its distribution of mass resulting from the break.

A boomerang does not have to be limited to two arms. Indeed, one of the easiest boomerangs to build is a four-blade design consisting of two rulers crossed and fastened at the center. The right kind of ruler has a curved top surface and a fairly flat bottom surface. You can attach two such crossed rulers either by wrapping a strong rubber band around them or by putting a bolt and nut through the center hole they usually have. Throw this type of boomerang the same way that you would a two-armed one. Take care to avoid being cut by sharp edges, and never use rulers that have metal edges.

A simple cross boomerang can be fashioned from a cardboard square about five inches on a side. Cut out a boomerang with three or four blades and twist them slightly so that the boomerang is not quite all in the same plane. Adding weight to the arms of a boomerang increases its range. With the cardboard boomerang it is easy to add weight by attaching paper clips to the end of the arms. This boomerang can be demonstrated in a classroom. If it has too much range for a classroom, you can decrease the range by increasing the twist on the arms so that the boomerang is less in the same plane or by bending the arms along a center line through their length. With the latter technique the arms have an exaggerated airfoil shape: at least one side is sharply convex but the other is not. As usual, throw the boomerang with the convex side toward you. By changing the arms from being almost flat to being more like an airfoil, you increase the lift on the boomerang: its path will be a tighter circle.

When you begin to throw your wood boomerang well, you might be tempted to catch it. The result may be a sharp blow to your fingers. If you are determined to catch a boomerang, hold your hands flat and slap them together to trap the boomerang as it hovers above the ground, still spinning, in the last stage of its flight. Keep your fingers away from the turning blades.

A four-blade design made with plastic rulers and rubber bands
The explanation of the return of a boomerang lies primarily in the cross-sectional shape of the arms and the fact that the boomerang spins. Without these two features a boomerang would behave like any other thrown stick. The cross-sectional shape gives the boomerang aerodynamic lift similar to the lift generated by some airplane wings. The spinning gives the boomerang stability. Through a bit of fortunate rotational mechanics the spinning also causes the axis about which the boomerang spins to rotate in much the same way that the spin axis of a top rotates about the vertical. The lift and the stability keep the boomerang up, and the rotation of the spin axis brings it back to the thrower.

Aerodynamic lift can be explained with a simple model of a classic airplane wing similar to the one I described in this department for February, 1978, to explain the lift of a kite. The classic airfoil has a flat bottom, a blunt front, a sharp rear and a convex top. Air passes around a wing faster along the top of the wing than along the bottom. The reason can be seen by visualizing the passing air as being of two kinds. One kind flows around the wing with no rotation in the stream and with the same speed on the top and the bottom of the wing. The other is a circulation cell: it flows to the rear over the top of the wing and to the front over the bottom. Such a circulation is created by a real wing because the air's viscosity and its adhesion to the surface of the wing force it into this pattern as it flows to the rear off the curved top surface.

In the superposition of the two idealized airstreams the two velocities add above the wing and subtract below it, with the result that the real air speed is greater above the wing than it is below it. The difference is important to the lift because the air pressure in the stream is inversely related to the speed of the stream. Hence the air pressure is less above the wing than below it, and the wing gets a push upward. (A real airplane wing can have a more complicated airflow pattern than this simple model implies. Moreover, when an airplane is traveling at high speed, some of the lift may come from the impact of the passing air on the underside of a wing that is inclined slightly upward in order to deflect the air downward.)

If the classic airfoil is inclined to the airstream in such a way that the airstream is more incident on the curved top side, the lift is of course less. Such an arrangement is termed a negative angle of attack. In a simple model the reduction of lift is due to the downward push the incident stream exerts on the top surface. One might also argue that lift is partially lost because the tendency for the air to circle about the wing is lessened and the speed of the air on the top side of the airfoil differs less from the speed on the bottom side.

Pattern of airflow over the arm of a flying boomerang
Conversely, if the airfoil is inclined so that the airstream is incident somewhat more on the flat bottom side than on the top, a situation that would be called a positive angle of attack, the lift increases because of the upward push from the airstream on the bottom side. The air drag also increases. If the angle is too large, the disadvantages of increased air drag outweigh the advantages of lift. The attack angle of the arms of the boomerang as they turn through the air is important to its flight.

Boomerang arms can have a variety of cross-sectional shapes, but most of them are similar in cross section to the classic airfoil. Usually this shape includes a blunt edge that turns into the air as the boomerang spins and a sharper edge that trails during the turn. One side is usually flat and the other convex. Variations on this basic form are numerous, however, and little systematic work seems to have been done on determining which shapes are best aerodynamically. Some boomerangs are actually flat on both sides but with their arms twisted so that the air is deflected as the arms turn through the wind.

The lift on a boomerang differs in a major way from the lift on the classic airplane wing. In the first stage of a flight the boomerang's "lift" is mostly horizontal, with only enough upward force to balance the weight of the device. Since the boomerang is spinning mostly about a horizontal axis, the curved sides of the arms spin in a plane that is almost vertical and the lift is almost horizontal. For the sake of simplicity in what follows I shall ignore the weight of the boomerang. I shall also assume a boomerang that is thrown outward by a right-handed thrower so that the plane of spinning is initially exactly vertical. The lift will be to the thrower's left, so that the boomerang begins to move to the left as it continues to spin in the vertical plane.

If this were the entire story, the boomerang would never come back. To see why it turns around and returns you must understand what else the lift does to the boomerang. In particular it is necessary to know how the torque due to the lift on the boomerang causes a precession of the spin plane.

Imagine that one of the boomerang's arms has spun to its highest possible position and the other arm is almost in its lowest possible position. (I am discussing the basic banana-shaped boomerang.) The upper arm is turning in the same direction in which the center of the boomerang is moving, whereas the lower arm is moving opposite to the motion of the center. The air passing the upper arm is moving faster (in relation to the arm) than the air passing the lower arm. Therefore more lift is generated on the upper arm than on the lower one. The part of the boomerang higher in the spin will always experience a greater lift and hence a greater push to the side than the part lower in the spin.

Positive and negative attack angles
My first thought was that the difference in horizontal lift (more lift on the upper arm than on the lower one) would cause the spin plane of the boomerang to tilt, thereby angling the lift downward (a disastrous effect). What actually happens, however, is that the difference in lift causes a rotation of the plane about a vertical axis. It is this rotation of the spin plane, commonly called precession, that brings the boomerang back.

To understand what causes the rotation you must examine the torque created by the lift. Take the center of the boomerang as the axis about which it is spinning. (Actually the center of mass around which a two-armed boomerang spins is likely to be well off center, but that does not alter the outcome of the argument.) Take the average lift on the upper arm as being directed horizontally outward from the center of the arm. Similarly, take the average lift on the lower arm as being also directed horizontally outward from the center of the arm. The torque created by one of these lifts, as measured from the center of the boomerang, is the product of the lift and the distance to where the lift is applied, that is, half the length of an arm. Since the upper arm has the greater lift, it also has the greater torque.

If the boomerang were not spinning this difference in torques would merely make the plane of the boomerang tilt over. Since the upper arm has the greater torque, the plane would tilt counterclockwise as seen by the person who has just thrown the boomerang. The fact that the boomerang is spinning, however, makes a big difference, because it then has angular momentum and the tendency to tilt the spin plane results in a rotation of the spin plane about the vertical axis.

Average lift on the upper and lower arms
Angular momentum is the product of the boomerang's rate of spin and a function involving the mass and the mass distribution of the device. For an example in another setting imagine yourself attempting to turn a merry-go-round holding several children. The force you apply tangent to the rim multiplied b the radius of the merry-go-round is the torque you are supplying. When you begin, the torque causes an angular acceleration of the merry-go-round; the spin increases from zero to some final value. How would you arrange the children in order to achieve a given angular acceleration with the least force? Intuitively you would place them near the center. Their mass is the same, of course, but their mass distribution with respect to the center of rotation is different. When the mass is nearer the center, the merry-go-round is easier to turn. The mass and its distribution are taken into account by the function known as the moment of inertia. The greater the mass or the farther from the center it is placed, the greater the moment of inertia and the greater the force you will have to supply in order to achieve a given angular acceleration.

Once the merry-go-round is spinning and you are no longer pushing on the rim, the apparatus has a certain angular momentum because it has spin and a moment of inertia. Angular momentum is usually represented by a vector pointing perpendicularly to the plane in which the object is spinning. Here the vector would be vertical. The direction (up or down) is chosen by convention as being the direction of the thumb on the right hand when the hand is held in a hitchhiker's pose with the fingers curled in the direction of the spin of the object.

The only way you could change the size or the direction of such a vector would be to apply another torque to the object. With a merry-go-round you could push on the rim again. (A convention for choosing how to draw a vector representing the change in angular momentum involves pointing the index finger of the right hand from the center of the rotation toward the place where the force is applied and pointing the middle finger in the direction of the applied force. If you make your thumb on that hand perpendicular to both fingers, it automatically points in the direction of the change in the angular momentum. The new angular-momentum vector is the combination of the old one and the vector representing the change.) With a merry-go-round that you had resumed pushing tangent to the rim the new vector would still be vertical but would be larger or smaller depending on whether your aim was to make the merry-go-round turn faster or slower.

How to determine angular momentum and a change in it
A boomerang that is spinning has two torques acting on the arms, one created by the average lift on the upper arm and one created by the average lift on the lower arm. Since the lift on the upper arm is greater, it determines what happens to the angular momentum, and so I shall ignore the lift on the lower arm. (The argument would not change even if I included the smaller lift.) Imagine that the boomerang is receding from you just after you have thrown it with your right hand. It is spinning in a vertical plane and has an angular-momentum vector pointing to your left. The average lift on the upper arm creates a torque that will change the direction of the vector as the boomerang continues to fly away.

To determine how the vector changes use your right hand, orienting the fingers and the thumb properly. With your index finger pointing from the center of the boomerang to the center of the upper arm and your middle finger pointing to your left in order to be in the direction of the lift on that arm, your outstretched thumb points toward you. Therefore the vector representing the change in angular momentum points toward you. Mentally combining the change vector and the original vector is best done from an overhead point of view. The change vector is perpendicular to the original one and gives a new vector rotated from the old one toward you. The size of the angular momentum is unaltered because the change vector is perpendicular to the old one. Only the direction of the angular momentum is changed, and it is rotated about a vertical axis to point more toward you.

This type of rotation of an angular-momentum vector is precession; it is seen when the axis of a top precesses about the vertical. Another common example of precession is seen in the turning of a motorcycle. The wheels of a motorcycle spin fast enough and have moments of inertia sufficiently large to make their angular momentum large. To turn the motorcycle you cannot just turn the handlebars, as you would when riding a bicycle. Instead you make the motorcycle lean in the direction of the turn. The torques then experienced by the motorcycle cause the angular-momentum vectors of the wheels to precess, turning the motorcycle as a whole.

How precession keeps the attack angle positive
During the precession of the spin plane of a boomerang the boomerang continues to travel along a path with a certain speed but is continuously deflected by the horizontal lift it experiences. The resulting path approximates a large circle. In a successful boomerang flight the spin plane will precess at the same rate at which the device circles in its path. Its angle of attack remains somewhat positive. This match is necessary in order to keep the arms at the proper attack angle.

Suppose the boomerang precesses too slowly. Then as it travels along its circular path its spin plane rotates about a vertical axis at a rate lower than the rate at which the boomerang as a whole travels along its path. When the spin plane lags behind, the attack angle becomes, increasingly negative and the boomerang loses lift.

If the spin plane precesses too quickly, it turns about a vertical axis faster than the boomerang as a whole travels along the large circular path. As a result the attack angle becomes increasingly positive until the spin plane is perpendicular to the oncoming airstream. The air drag would surely ruin the flight by then.

The match between the precessional rate and the rate at which the boomerang travels along the large circular path is not critical and is in fact somewhat automatic, since both rates depend on the lift. Throw your boomerang, sand down and reshape the arms and throw it again until you come near the match and the boomerang returns. I know of no sure way to remedy a persistently unsuccessful boomerang.

The circular path of the boomerang is independent of the speed with which you throw it. Only the moment of inertia and the cross-sectional shape of the boomerang determine the radius of the path. With a given boomerang you will therefore achieve the same large circle (for the same throw of the boomerang in the vertical plane I have been assuming) regardless of how hard you throw the device (provided, of course, you throw it hard enough so that it has sufficient speed to complete its journey). If you want to change the size of the circle, you must ordinarily choose a different boomerang with a different moment of inertia or cross-sectional shape. Next month, however, I shall explain how you can also add ballast to the arms in order to increase their moment of inertia. This technique is used by boomerang throwers intent on breaking distance records.

A Frisbee flies in a quite similar way. It has a curved top surface and is launched with a flick of the wrist to give it spin. The Frisbee gains lift by virtue of the impact of the air or by the difference in air speed across the top and bottom surfaces. A Frisbee properly thrown in an almost vertical plane will return to the thrower the way a boomerang does. Usually, however, a Frisbee is launched to curve slightly to another person, so that the thrower orients the spin plane to provide just enough horizontal lift to achieve the curve.

Both a boomerang and a Frisbee can be skipped across the ground without destroying the flight. Imagine a Frisbee skimming just above the ground with its leading edge tipped slightly downward. That edge then strikes the ground. The force from the ground at the contact point puts a torque on the Frisbee and changes the angular momentum, but because the change vector is almost perpendicular to the original angular-momentum vector the new angular-momentum vector is just a rotation of the original one. The angular momentum does not change appreciably in size, only in direction. Therefore the spin of the Frisbee is not much slowed; the device is merely reoriented and then goes sailing off in a new direction.


BOOMERANGS: MAKING AND THROWING THEM. Herb A. Smith. Gemstar Publications, Arun Sports, Littlehampton, Sussex, 1975.

THE PHYSICS OF FRISBEE FLIGHT. Jay Shelton in Frisbee: A Practitioner's Manual and Definitive Treatise, edited by Stancil E. D. Johnson. Workman Publishing Co., 1975.


How to build microgram electrobalances

by Shawn Carlson

MICROGRAM BALANCES ARE CLEVER devices that can measure fantastically tiny masses. Top-of-the-line models employ an ingenious combination of mechanical isolation, thermal insulation and electronic wizardry to produce repeatable measurements down to one tenth of a millionth of one gram. With their elaborate glass enclosures and polished gold-plated fixtures, these balances look more like works of art than scientific instruments. New models can cost more than $10,000 and often require a master's touch to coax reliable data from background noise.

Figure 1: SCAVENGED PARTS from an old galvanometer can become a delicate microbalance. The circuit schematic (opposite page) shows how to power the galvanometer coil.

But for all their cost and outward complexity, these devices are in essence quite simple. One common type uses a magnetic coil to provide a torque that delicately balances a specimen at the end of a lever arm. Increasing the electric current in the coil increases the torque. The current required to offset the weight of the specimen is therefore a direct measure of its mass. The coils in commercial balances ride on pivots of polished blue sapphire. Sapphires are used because their extreme hardness (only diamonds are harder) keeps the pivots from wearing. Sophisticated sensing devices and circuitry control the current in the coil-which is why microgram electrobalances are so pricey.

And that is good news for amateurs. If you are willing to substitute your eyes for the sensors and your hands for the control circuits, you can build a delicate electrobalance for less than $30.

George Schmermund of Vista, Calif., made this fact clear to me. For more than 20 years, Schmermund has run a small company called Science Resources, which buys, repairs and customizes scientific equipment. Although he may be an austere professional to his clients, I know him to be quite the free spirit who spends time in the business world only so he can make enough money to indulge his true passion-amateur science.

Figure 2: Circuit schematic

Schmermund already owns four expensive commercial microgram balances. But in the interest of advancing amateur science, he decided to see how well he could do on the cheap. His ingenious ploy was to combine a cheese board and an old galvanometer, a device that measures current. The result was an electrobalance that can determine weights from about 10 micrograms all the way up to 500,000 micrograms (0.5 gram).

The precision of the measurements is quite impressive. I personally confirmed that his design can measure to 1 percent masses exceeding one milligram. Furthermore, it can distinguish between masses in the 100-microgram range that differ by as little as two micrograms. And calculations suggest that the instrument can measure single masses as slight as 10 micrograms (I didn't have a weight this small to test).

The crucial component, the galvanometer, is easy to come by. These devices are the centerpiece of most old analog electric meters-the kind that use a needle mounted on a coil. Current flowing through the coil creates a magnetic field that deflects the needle. Schmermund's design calls for the needle, mounted in the vertical plane, to act as the lever arm: specimens hang from the needle's tip.

Electronic surplus stores will probably have several analog galvanometers on hand. A good way to judge the quality is to shake the meter gently from side to side. If the needle stays in place, you're holding a suitable coil. Beyond this test, a strange sense of aesthetics guides me in selecting a good meter. It is frustratingly difficult to describe this sense, but if I'm moved to say, "Now this is a beautiful meter!" when I look it over, I buy it. There is a practical benefit to this aesthetic fuzziness. Finely crafted and carefully designed meters usually house exquisite coils that are every bit as good as the coils used in fine electrobalances, sapphire bearings and all.

To build the balance, gently liberate the coil from the meter housing, being careful not to damage the needle. Mount the coil on a scrap sheet of aluminum [see illustration in Figure 1]. If you can't use aluminum sheet metal, mount the coil inside a plastic project box. To isolate the balance from air currents, secure the entire assembly in a glass-covered cheese board, with the aluminum sheet standing upright so that the needle moves up and down. The two heavy guard wires cannibalized from the meter are mounted on the aluminum support to constrain the needle's range of motion.

Figure 3: CALIBRATION OF BALANCE is accomplished by plotting known masses against the amount of voltage needed to life each weight.

Epoxy a small bolt to the aluminum support, just behind the needle's tip. The needle should cross just in front of the bolt without touching. Cover the bolt with a small piece of construction paper, then draw a thin horizontal line across the center of the paper. This line defines the zero position of the scale.

The specimen tray that hangs from the needle is merely a small frame home-fashioned by bending noninsulated wire. The exact diameter of the wire is not critical, but keep it thin: 28-gauge wire works well. A tiny circle of aluminum foil rests at the base of the wire frame and serves as the tray pan. To avoid contamination with body oils, never touch the tray (or the specimen) with your fingers; rather always use a pair of tweezers. To energize the galvanometer coil, you'll need a circuit that supplies a stable five volts [see illustration below]. Do not substitute an AC-to-DC adapter for the batteries unless you are willing to add filters that can suppress low-frequency voltage fluctuations.

The device uses two precision, 100-kilohm, 10-turn, variable resistors (also called potentiometers or rheostats)-the first to adjust the voltage across the coil and the second to provide a zero reference. A 20-microfarad capacitor buffers the coil against any jerkiness in the resistors' response and helps in making any delicate adjustments to the needle's position. To measure the voltage across the coil, you'll need a digital voltmeter that reads down to 0.1 millivolt. Radio Shack sells handheld versions for less than $80. Using a five-volt power supply, Schmermund's scale can lift 150 milligrams. For larger weights, replace the type 7805 voltage-regulator chip with a 7812 chip. It will produce a stable 12 volts and will lift objects weighing nearly half a gram.

To calibrate the scale, you'll need a set of known microgram weights. A single high-precision calibrated weight between one and 100 micrograms typically costs $75, and you'll need at least two. There is, however, a cheaper way. The Society for Amateur Scientists is making available for $10 sets of two calibrated microgram weights suitable for this project. Note that these two weights enable you to calibrate your balance with four known masses: zero, weight one, weight two and the sum of the two weights.

To make a measurement, begin with the scale pan empty. Cover the device with the glass enclosure. Choke down the electric current by setting the first resistor to its highest value. Next, adjust the second resistor until the voltage reads as close to zero as possible. Write down this voltage and don't touch this resistor until you have finished all your measurements. Now turn up the first resistor until the needle sinks down to the lower stop, then turn it back so that the needle returns to the zero mark. Note the voltage reading again. Use the average of three voltage measurements to define the zero point of the scale.

Next, increase the resistance until the needle comes to rest on the lower wire support. Place a weight in the tray and reduce the resistance until the armature once more obscures the line. Record the voltage. Again, repeat the measurement three times and take the average. The difference between these two average voltages is a direct measure of the specimen's weight.

Once you have measured the calibrated weights, plot the mass lifted against the voltage applied. The data should fall on a straight line. The mass corresponding to any intermediate voltage can then be read straight off the curve.

Schmermund's balance is extremely linear above 10 milligrams. The slope of the calibration line decreased by only 4 percent at 500 micrograms, the smallest calibrated weight we had available. Nevertheless, I strongly suggest that you calibrate your balance every time you use it and always compare your specimens directly with your calibrated weights.

To receive the two calibrated weights, send $10 and a self-addressed, stamped envelope to the Society for Amateur Scientists, 4951 D Clairemont Square, Suite 179, San Diego, CA 92117. For more information about this project, send $5 to the address above or download it for free from the SAS Web page at http:// or Scientific American's area on America Online.

How to build an electronic neuron

by John Iovine

ARTIFICIAL NEURAL NETWORKS ARE electronic systems that function and learn according to biological models of the human brain. Typically such networks are implemented in computers as programs, coprocessors or operating systems. By mimicking the vast interconnections of neurons, researchers hope to mirror the way the brain learns, stores knowledge and responds to various injuries. The networks might someday even be a basis for future intelligent thinking machines [see "Will Robots Inherit the Earth?" by Marvin Minsky, page 108]. They may also help to surmount the barriers faced by standard programming, which fails to perform in real time some tasks the human mind considers simple, such as recognizing speech and identifying images.

Figure 1: HARD-WIRED NEURAL NETWORK tracks the sun by keeping two photosensors equally lit. A motor that runs too quickly may need to be coupled to a larger gear (inset).

In an artificial neural network, objects called units represent the bodies of neurons. The units are connected by links, which replace the dendrites and axons The links adjust the output strength of the units, mimicking the different strengths of the connections between synapses, and transmit the signal to other units. Each unit, like a real neuron, fires only if all the input signals routed to it exceed some threshold value.

The primary advantage of such an architecture is that the network can learn. Specifically, it can adjust the strength, or weight, of the links between units. In so doing, the links modify the output from one unit before feeding the signal to the next unit. Some links get stronger; others become weaker. To teach a network, researchers present so-called training patterns to the program, which modify the weight of the links. In effect, the training alters the firing pattern of the network [see "How Neural Networks Learn from Experience," by Geoffrey E. Hinton; SCIENTIFIC AMERICAN, September 1992].

What I describe here is the construction of a simple, hard-wired neural network. Using a motor, this circuit follows the motion of a light source (such as the sun). All the parts are readily available from electronic hobby shops such as Radio Shack.

The operation of the circuit is simple, particularly because it relies on only one neuron. The neuron is a type 741 operational amplifier (op-amp), a common integrated circuit. Be sure the op-amp comes with a pin diagram, which identifies the connection points on the op-amp by number.

Two cadmium sulfide photocells act as neural sensors, providing input to the op-amp. The resistance of these components, which are about the size of the hp of your little finger, changes in proportion to the intensity of light. With epoxy or rubber cement, glue the photocells a couple of centimeters apart on a thin, plastic board that is approximately three centimeters wide by five centimeters long. Then affix a similarly sized piece of plastic between the cells so that the assembly assumes an inverted T shape. This piece must be opaque; I painted mine black.

The rest of the circuit should be built on a stationary surface a few centimeters from the photosensor assembly. A breadboard-a perforated sheet of plastic that holds electronic components-will help keep the connections tidy.

You will also need a power supply: a couple of nine-volt batteries will do the job. Connect the batteries together by wiring the positive terminal of one battery to the negative end of the other (in effect, grounding them). This configuration leaves open one terminal on each battery, thereby creating a bipolar power supply. Four components need to draw electricity: the two photocells, the op-amp and the motor. Connect these parts in parallel to the batteries. For convenience, you may wish to wire in an on-off switch.

On the schematic [see illustration below], you will notice several resistors. They act to stabilize the amount of current that flows through the circuit. A 10-kilo-ohm potentiometer-basically, a variable resistor-is connected to one of the photocells. This component regulates the voltage received by the op-amp-that is, it adjusts the weight of the link.

Figure 2: CIRCUIT SCHEMATIC of the neural network shows the necessary connections. The type 741 operational amplifier acts as the neuron.

Hook up the photosensors so that they are connected to pin numbers 2 and 3 of the op-amp. The power supply goes to pins 4 and 7. The output signal leaves the op-amp at pin number 6 and travels to two transistors. One, labeled Q1 on the schematic, is a so-called NPN type; the other, Q2, is a PNP type. These transistors activate the motor and, in some sense, can be looked on as artificial motor neurons.

The motor is a low-voltage, direct-current type. The one I used was a 1 2-volt, one-revolution-per-minute (RPM) type. If your motor's RPM is too high, you will need to couple a large gear to it to reduce the speed [see Figure 1]. The motor should have a shaft about six centimeters long. To extend mine, I inserted a stiff plastic tube over the end of the motor shaft.

To train the circuit, expose both photocells to equal levels of light. A lamp placed directly above the sensors should suffice. Adjust the potentiometer until the motor stops. This process alters the weight of the signal, so that when both photosensors receive equal illumination, the op-amp generates no voltage. Under uneven lighting conditions, the output of the op-amp takes on either a positive voltage (activating the NPN transistor) or a negative voltage (triggering the PNP transistor). The particular transistor activated depends on which sensor receives the least amount of light.

To test the circuit, cover one photocell; the motor should begin rotating. R should stop once you remove the cover. Then block the other photocell. The motor should begin rotating in the opposite direction.

Now glue the photosensor assembly to the shaft of the motor so that the photocells face up. Illuminate the sensors from an angle. If the motor rotates in the wrong direction (that is, away from the light), reverse the power wires to the motor. You may have to cut down on the amount of light reaching the photocells; full sun will easily saturate the sensors. Just cover the photocells with a colored, translucent piece of plastic.

As long as the sun is directly aligned with the two photocells, exposing them to equal amounts of light, the inputs to the neuron balance out. As the sun moves across the sky, the alignment is thrown off, making one input stronger than the other. The op-amp neuron activates the motor, realigning the photocells. Notice that this neural circuit tracks a light source without relying on any equations or programming code.

The circuit has immediate practical applications in the field of solar energy. For example, it can be hooked up to solar-powered cells, furnaces or water heaters to obtain the maximum amount of light input.

You can also modify the device in a number of ways. For instance, you can hook up a second network so that you can track a light source that moves vertically as well as horizontally. ambitious amateurs might try replacing the photocells with other types of sensors, such as radio antennae. Then you can track radio-emitting satellites across the sky. Photocells sensitive to infrared energy could be used to track heat sources-the basis for some types of military targeting. Plenty of other modifications are possible, but don't expect your neuron ever to achieve consciousness.

More intricate examples of the circuit described in the article demand fairly complicated hard-wiring. Complex variations are therefore perhaps best constructed as software. I wrote a program in BASIC that emulates an early neural network-the Perceptron, created in 1957 by Frank Rosenblatt of Cornell University. The Perceptron learns to identify shapes and letters. This software, as well as a few other artificial neural network programs, is available on an IBMcompatible disk for $9.95, plus $5.00 for postage and handling, from Images Company, R O. Box 140742, Staten Island, NY 10314, (718) 698-8305.


THE THREE-POUND UNIVERSE. Judith Hooper and Dick Teresi. Macmillan Publishing, 1986.

FOUNDATIONS OF NEURAL NETWORKS MONDO PRIMER. Tarun Khanna. Addison-Wesley Publishing, 1990.


How to build homemade high-precision thermometer

by Shawn Carlson

Last month I described how to build a triple-point cell, a device that reproduces the unique temperature, defined to be exactly 0.01 degree Celsius, at which water can exist with its solid, liquid and vapor phases all in equilibrium. The cell can be used for calibrating state-of-the-art thermometers, but few amateur scientists can afford such expensive instruments, which cost thousands of dollars.

Fortunately, George Schmermund, the creative genius from Vista, Calif., who developed our triple-point cell, has also designed a thermometer capable of measuring temperature to within a few thousandths of a degree C. What is more, you can build this remarkable instrument for less than $100.

Schmermund's thermometer uses something called a resistance temperature detector (RTD), which relies on the fact that the resistance of platinum changes with temperature in a precisely known way. For each degree C of temperature change, these sensors typically change their resistance by 0.00385 ohm per ohm of resistance. For example, if your RTD has a resistance of 100 ohms, each degree C change in temperature will alter the resistance by 0.385 ohm. So if you know the probe's resistance at a particular temperature, such as the triple point of water, you can then convert any measured resistance into a corresponding temperature.

EXACT TEMPERATURE MEASUREMENTS to within millidegrees can be made with a thermometer that relies on a resistance temperature detector (RTD)--a sensor that exploits how the electrical resistance of platinum changes as the material becomes hotter or colder. The relation is linear and is given by the equation shown (above), where a (Alpha) is typically 0.00385 and RTP is the resistance of the sensor at 0.01 degree Celsius: the triple-point temperature of water. A digital multimeter measures the RTD's resistance.

In the past, RTDs were always made of wire. Because the wire had to be thick enough to withstand the manufacturing processes and because a larger-diameter wire has less resistance than a smaller one made of the same material, the operating resistance was limited to about 100 ohms. Recently, though, a new breed of RTDs has been constructed by laying an ultrathin platinum coating on a ceramic substrate. The resistance of some of these devices tops 2,000 ohms. You'll find a smorgasbord of these marvels in the catalogue of Omega Engineering in Stamford, Conn. (; 800-826-6342). For this project you'll need a model like the F3141, a small, unencapsulated 1,000-ohm unit that sells for $19.

These new RTDs can bring exquisite sensitivity into the home-based laboratory. Using a high-quality handheld digital multimeter that can measure 1,000 ohms of resistance to within 0.02 ohm, amateurs can now resolve temperatures to within 0.005 degree C, or 0.009 degree Fahrenheit. That performance bests liquid-filled thermometers by 20 times and trumps any thermocouple by a factor of 10.

And you can do much better. In practice, the sensitivity of an RTD-based thermometer is limited by how accurately you can determine its resistance, which is measured by observing the voltage drop associated with a known current. With a typical digital multimeter, the lead wires are part of the circuit, and so their resistance affects the results. This error can be eliminated by measuring the voltage drop directly across the resistor with an independent set of wires. Such instruments, called four-wire ohm meters, have separate inputs for a current source and a volt meter.

Hewlett-Packard's snazzy HP 34401A multimeter (priced at $995) uses the four-wire technique to measure 1,000 ohms to within 0.001 ohm. And top- of-the-line instruments costing $30,000 apiece enable professionals to resolve temperature differences as slight as 10 millionths of a degree C. In such a four-wire configuration, an RTD-based thermometer is called a standard platinum resistance thermometer (SPRT).

To build Schmermund's thermometer, contact a local glassblower to purchase a Pyrex tube 30 centimeters (12 inches) long and eight millimeters (0.3 inch) in diameter. At one end of the tube, have the glassblower form a receptacle that is five centimeters (two inches) long for the RTD sensor.

Next, attach lead wires to the sensor. If you solder the leads or use wires insulated with plastic, you'll be restricted to temperatures below the melting point of those substances. That's not a problem for many applications. To allow the maximum possible range of temperatures, however, Schmermund spot-welds the RTD to bare 10-mil nickel wires that he then insulates in thin Pyrex sleeves. He gets these sleeves in 46- centimeter lengths from a local glassblower, but capillary tubes, which are available from any scientific supply house, work equally well when strung on the wire like beads on a necklace.

For a thermometer that will be used with a four-wire ohm meter, Schmermund bundles four of his long tubes and delicately tapes them together at one end. He then bends two one-meter lengths of nickel wire in half and threads each half through a different tube from the untaped end. Finally, he spot-welds the RTD to the bends in the two wires. (Note: If you will not be making four-wire measurements, simply connect one wire to each of the RTD leads.)

To secure the insides of the device and to thwart convection currents from forming, Schmermund packs the instrument with tiny glass beads that are only about 25 microns in diameter. These are expensive and must be purchased from a scientific supply house. Fortunately, fine silica sand (grit 30 or greater) also does the job. You can purchase a 23-kilogram (50-pound) sack from a hardware store for just a few dollars.

Because any moisture that becomes trapped inside the thermometer will distort your readings, all water must be driven from both the filler and the glassware before assembly. Bake everything, including the entire sensor assembly, at 250 degrees F for approximately two hours.

You must complete the next steps while everything is hot, so be sure to exercise the proper care by wearing gloves, an eye shield and protective clothing. Secure the large tube in a vise. A clean rag wrapped around each jaw will allow you to hold the glass tube firmly without breaking it. Insert the RTD assembly into the tube and use a small glass funnel to pour in enough of the desiccated sand to cover the sensor completely. Lift the assembly just a bit to make sure the RTD is suspended about two millimeters above the bottom of the well, without it touching the glass wall. Remove the tape and slowly fill the tube with hot sand to within about half a centimeter from the top, stopping frequently to tap the glass with a pencil to consolidate the material.

Hermetically seal the thermometer by topping off the sand with glue from a hot-glue gun. If you're using uninsulated wires, heat them with a hair dryer for a few seconds before the adhesive sets so that the wires will seat themselves into the glue.

To minimize signal interference, connect the probe to your ohm meter through a stereo microphone cable, which consists of two twisted pairs of wire shielded inside a metal sheath that you must ground. Use a four- wire terminal strip to connect each twisted pair across the device. Solder the wires and protect the strip inside a plastic canister from a roll of 35-millimeter film.

This homemade instrument, which is functional up to about 400 degrees C, can open up fascinating avenues of research. Although the device is a bit cumbersome for fieldwork, you can use it for accurately calibrating other thermometers. In the laboratory, it will also help you probe the nature of phase transitions and measure the strength of chemical bonds (for ideas, see the March 1996 Amateur Scientist). With a little imagination, this thermometer can become a powerful weapon in your arsenal of research techniques.

How to build a combat robot

Experience the excitement of building your own champion battling bot!

Build a powerful and invincible robot--for full-blown competition or just for fun--using this authoritative robot resource. This team of experts gives you an inside look at the innovative new world of robotic combat, explaining the origins of the sport as well as all the elements that go into constructing a fighting robot. You'll learn technical basics from motors and wiring to locomotion, and read builders' true stories from the front lines of robot competition. Whether or not you're mechanically-minded, you'll find this book both entertaining and informative. Fully capturing the spirit of the sport, this detailed guide shows you how an imagination and a few bits of metal can start you on your way to constructing your own champion bot.

  • Plan, design, and build your own battling robot
  • Discover how electric motors work and choose the right type for your design
  • Learn about both wheeled and walking robots
  • Incorporate creative weapons and armor on your bot--learn the pros and cons of spinning hammers, jabbing spikes, buzzing saw blades, and more
  • Minimize radio interference--so you can effectively control your bot to do what you want, when you want
  • Understand what goes into building fully autonomous robots
  • Save money on parts and equipment by enlisting help from company sponsors
  • Read true stories from combat roboteers and learn which types of constructions work and which don't

About the Author
Pete Miles (Graham, WA) has worked in robotics for over 12 years. He holds a Master's degree in mechanical engineering and is currently working as Senior Research Engineer in advanced machining technologies for Ormond LLC in Kent, WA. Miles is active with the Seattle Robotics Society and was recently appointed to the SRS Board of Directors.

Tom Carroll (Eastsound, WA) has been involved with robotics for over forty years. He built his first robot at age 14, and later worked as a robotics engineer for Rockwell International for nearly thirty years. He built the robots that were featured in the movies Revenge of the Nerds and Buck Rodgers in the 25th Century, and founded his own company, Universal Robot Systems, which focuses on personal robots to assist the elderly and disabled.

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How to make biodiesel

What is biodiesel? Simply put, it is diesel fuel that is made from vegetable oil. It will run in any unmodified diesel engine. It has many advantages over petroleum diesel fuel such as: 1) It burns cleaner 2) It has a higher cetane rating (less knocking) 3) It has better lubricity 4) You can make it yourself from used vegetable oil (a waste product) often for less than the cost of petroleum diesel.
You will need the following things to make your first batch:
  1. At least 1 Litre (1.1 Quart) vegetable oil. Canola oil, corn oil, soybean oil, etc will suffice.
  2. A variable speed blender with a slow speed option. Use one with a glass pitcher only. The methanol that is used in this process will "eat" a plastic pitcher. Make sure that this blender will never be used for food products again.
  3. A scale that will accurately measure 3.5 grams (.12 oz). I use a triple beam balance available through Edmund Scientific. Search the site for the keyword "balance". A good scale will cost between $100 and $200 and is a good investment. However, if you are on a budget, you can get the Edmund Scientific "Carry-With-You Twin Beam Balance" (Stock Number: CR30360-28) which will weigh up to 4 grams. This costs $25.
  4. 1 bottle Red Devil Lye Drain Cleaner (Sodium Hydroxide) available from you local hardware store. Make sure the label says "contains sodium hydroxide". Most other drain cleaners are chlorine (Calcium Hypochlorite) based and will NOT work! Notice: Lye is poisonous! Take all necessary safety precautions!!
  5. At least 200 milliliters (6.8 fl. oz) of methanol (Methyl Alcohol or "Wood" Alcohol). Methanol is widely available in 12 oz. quantities as "gas tank antifreeze" in auto parts stores, hardware stores and even some grocery stores. Popular brands include "Heet" and "Pyroil". Read the label carefully and make sure it says "contains methanol"! Many gas line antifreeze products contain isopropyl alcohol or "isopropanol" and will NOT work! Methanol is available in larger quantities as racing fuel through some racetracks that cater to drag racers and some "high performance" auto parts stores. Keep in mind that Methanol is both poisonous and flammable. Take all necessary safety precautions!!
  6. A glass container that is marked for 200 milliliters (6.8 fl. oz). We like to use a beaker.
  7. A glass or plastic container that is marked for 1 liter (1.1 Quart)
  8. A wide mouth glass or plastic container that will hold at least 1.5 litres
  9. A common spoon (preferably plastic or stainless steel).
  10. Safety Glasses and Rubber Gloves! Methanol and Lye are extremely poisonous and must not come into contact with skin or eyes! Methanol is a poison that attacks the eyes (ocular nerves) even if it comes into contact with your hands. Use extreme care when blending the methanol and lye, as the blender can spash the chemicals around. Put on your glasses and gloves BEFORE opening the chemicals! Do your work close to a sink or hose, or have a bucket of water handy to wash any part of your body immediately if it comes in contact with these chemicals.
Get organized in a well lit, well ventilated area! This process is best done at or above room temperature (70 degrees F or 21 Degrees C). Temperatures lower than 60 F or 15 C may cause an incomplete reaction. Plan for spills by spreading paper or plastic on your work surface. Put your safety glasses and gloves on before opening any chemicals!

What you work
  • Measure 200 milliliters (6.8 fl. oz) of methanol
  • Pour the methanol into the blender. Notice the glass pitcher on the blender.
  • Weigh out 3.5 grams of lye on your scale. Notice that we use a white piece of plastic to hold the lye. The weight of the plastic is 4 grams, so we set the scale to 7.5 grams.
  • Turn the blender on "slow" speed and slowly add the lye to the methanol. You now have a mixture called "sodium methoxide". The methoxide must be used right away to make biodiesel. Do not plan on making a large batch of methoxide and storing it for use later. It loses its potency over time.
  • After the Lye has completly dissolved into the methanol (about 2 minutes), add 1 liter of vegetable oil to the blender. Blend on low speed for 20 to 30 minutes. The ideal speed for this process just barely creates a vortex or "tornado" in the oil without spashing the mixture around or frothing it up.
  • After the blending is complete, pour the mixture into the wide mouth jar. It is advisable to label all containers used in this project as "POISON"! And of course, keep all of this stuff away from children!
  • After about 30 minutes to 1 hour, you will notice a layer of darker colored glycerin settling to the bottom of the container. The lighter layer on top is biodiesel. Wait another few hours for complete settling. At that point, you can carefully pour off the lighter biodiesel from the top and discard the glycerin (or save the glycerin to use in soapmaking). An alternative would be to use a pump to remove the biodiesel from the jar. You are done!
It is always wise to use a "diesel fuel filter/water separator" with any diesel engine. These are available through some auto parts stores or A good model is the Racor 120AS diesel fuel filter/water separator (West Marine # 411348).

Biodiesel has a solvent effect on natural rubber hoses and seals. While newer diesel engines have polymer hoses and seals (such as Dupont's "Viton" brand), older engines may need to be outfitted with new hoses and/or seals made of viton. Since most diesel injector pumps don't have rubber parts directly in contact with the fuel, it is usually easy to replace hoses and seals without any major dissasembly. A fuel mixture of 20% biodiesel and 80% petroleum diesel (called "B20") will have no effect on older natural rubber hoses.

Biodiesel will "cloud" at temperatures below 55 degrees F (13 degrees C). While this "clouding" is easily reversible by raising the temperature of the fuel again to above 55 degrees, it may cause temporary clogging of your fuel system, thus stopping your engine. Petroleum diesel fuel (Diesel #2) can be used down to -10 degrees F (-24 degrees C). It is advisable to use a blend of at least 50% petroleum diesel with your biodiesel if you are going to be operating in cold weather. You can experiment with different blends of biodiesel and petroleum diesel to determine what works best. Simply mix up batches of fuel with different ratios of petroleum diesel and biodiesel in glass jars and put in a freezer. Use a thermometer to determine the temperature of the fuel. Periodically check on the fuel to determine at what temperature it gets cloudy. This temperature is the "cloud point". It is best to determine this point at home before you head out on the road and get stranded in a snowstorm because your mixture is too rich in biodiesel. Of course, if you are going to be operating during the warm months, or in a warm climate, you can use 100% biodiesel with no problems.

How to build simple seismograph

A seismograph is a device used for recording earth tremors. Basically, it is a heavily weighted horizontal rod (pendulum) suspended from a pole. This rod is free to swing from side to side if the earth shakes. One end of the rod rests against the pole, while the other holds a pen or stylus. This stylus marks a slowly moving roll of paper. If there is no shaking, the passing paper is marked with a straight line. If there is a tremor, the paper is marked with a squiggly line.

The waves that reach the seismograph are, in order: the P or primary waves, which are caused but compression of rock, and which travel straight through the earth; S or secondary waves, which are shear waves caused by rock shaken from side to side; and L waves, which are surface waves caused by rolling motions of the surface. L waves, which travel along the surface, are the last to arrive. They are also the most destructive. Since these waves travel at different speeds, seismologists can pinpoint the origin of an earthquake (epicenter) by comparing their arrival times.

A simple seismograph, as shown here, is easy to build and will record local vibrations such as passing trucks or people walking past it. Later in the article are modifications to this design that will make an instrument sensitive enough to record distant earthquakes. The text and photos for this model are from Science Equipment, by William Moore, 1962. You will need:
  • 1 piece steel pipe, threaded at least at one end, 1 inch by 3 feet (25 mm by 100 cm)
  • 1 steel floor flange 3 1/2 inches (88mm) in diameter (should fit the pipe)
  • 1 steel rod 1/4 inch by 24 inches (6mm by 60cm)
  • 1 piece steel wire app. 3 feet (100 cm) long
  • 1 wind up alarm clock
  • 1 piece of wood, 3/4 inches by 1 foot by 3 feet (20mm by 305mm by 915mm)
"Begin by screwing the flanged plate to one end of the large board. Next drill holes through the top end of the pipe for the wire. Also drill a hole for the wire at about one inch (25mm) from the end of the steel rod. The steel rod must be pointed at both ends, so place it in a vise and file on alternate sides of the rod. After this is done, drill a small dimple about six inches (150mm) above the threaded end of the pipe in which one pointed end of the steel rod may rest Attach the wire to the end of the pipe and the end of the rod next, and after screwing the pipe into position tightly, place the pointed end of the rod in the dimple. Now wire some heavy weights to the steel rod. These may be lead blocks as shown here, or bricks, or even sash weights.

The recording device should be prepared next. This is done by removing the glass or plastic cover on the clock face and cementing a white cardboard disc to the (hour) hand of the clock. Use a candle or small lamp to cover the disc with soot. Now mount the clock on a piece of scrap wood, so that the point of the steel rod barely touches the disc at the nine o'clock position on the dial. Experiment with the position of the clock until the correct location is found before fastening it permanently. For greater accuracy, be sure to clamp the baseboard to the bench or table."

Once the basic design is understood, it is possible to design and construct a much more sensitive seismograph. There is a great deal of leeway for experimentation. A few tips are:
  • The base should be solid and positioned on the ground. A large paint drum half filled with concrete works well; a concrete umbrella stand works even better.
  • The upright support should be very solid. Metal pipe or angle iron works well as long as it can stand upright and remain very steady. The post should be anchored firmly in the concrete base. Three or four feet high (90-125 cm) should be sufficient.
  • Any rigid length of metal can be substituted for the steel rod arm (pendulum) in the above plan, as long as it rigid enough not to bend when suspended from the upright. A thin strip of metal can be folded to make a strong, rigid channel. It should be about 3/4 long as the upright is tall. The end of the rod that contacts the upright comes to a sharp point. Ideally, the point of contact on the upright should be a bearing, which can be adjusted in or out for fine-tuning. This point of attachment should be about 6 inches (150mm) up from the base (this will depend on how high your recording apparatus is positioned).
  • The wire supporting the arm/pendulum attaches to the top of the upright. It is best to make this attachment adjustable with an arrangement of angle irons, nuts, and washers so that the wire and pendulum can be easily raised, lowered, and moved from side to side in small increments.
  • The wire attaches to the arm/pendulum at about six inches from the upright. The weight should lie near this point of attachment. A metal food can filled with plaster is convenient to work with; the arm/pendulum can pass through it, and the wire can attach to the can itself.
  • The other end of the arm holds the recording stylus. What is used here depends on the material being recorded on. A needle works well for smoked metal, aluminum foil, or glass. If paper is the recording material of choice, pens must be chosen that will write with light pressure, but will not readily dry out. The type of pens used in plotters are one option. Whatever is used, it must be remembered that it will be striking the recording surface very lightly.
  • There is more room for improvisation with the recorder than with the rest of the seismograph. The clock dial method above works well; there is an article on a cylinder type phonograph in these pages that may give the adventurous designer an idea. If you do it this way, though, remember that the drum must rotate easily if it is to be turned by the minute hand post of an electric or wind-up clock. The drum should rotate away from the stylus. It is also possible to record on a roll of adding machine paper using two rotating drums. The recorder should be firmly set so that it does not get knocked around easily.

How to build vacuum engine

Power is transmitted in everyday life most often by electricity. There are other means of power transmission such as high-pressure air, high-pressure hydraulic oil, and, on industrial sites, steam. However, electricity dominates: it is the most versatile form of energy. It can be converted efficiently to any other form of energy, something that is not true of other types.

Most kinds of power transmission have a certain degree of tangibility. The rotating propeller shaft of a large truck leaves no doubt that considerable power flows from the engine at the front to the axles at the rear. Wander near a highpower electric system, and you’ll readily hear the low but insistent hum at the 50 or 60 Hz line frequency; sometimes you’ll even feel the hairs on your body react. The apparently inert wires and cables of high-power electric systems can produce huge and mortally dangerous flashes and sparks if they are disturbed. Similarly noisy and spectacular gas jets signal the presence of even small leaks in compressed air or steam systems.

By comparison, the transmission of power through a vacuum in a pipe seems a peculiarly intangible concept. How can power be apparently transmitted by nothing? But in this project we show that a vacuum can indeed transmit power, and that we can demonstrate a motor rather like an old-fashioned steam engine, an engine that can turn the power transmitted by a vacuum in a pipe into mechanical energy.

The Industrial Revolution that transformed the Western world, starting about 1700, needed mechanical power. At first, increased use and more efficient designs of watermills and windmills could provide that power. But it gradually became evident that the continuous power which steam could provide was going to be needed. It is easy to appreciate the expansive force of steam when you see a kettle boil. However, none of the early steam engines used that expansive power. Instead they used atmospheric pressure (they became known later as “atmospheric” engines), with the steam being used to create a vacuum so that the atmosphere could push a piston. We might today, perhaps less accurately, call them vacuum engines.

There have been times when vacuum power transmission has been used. Perhaps the first example was the system used by Matthew Boulton and his partner James Watt. Near the Boulton and Watt engine factory in Birmingham, England—the world’s first engine factory—was the Boulton and Watt mint, a coin factory operated by one of the company’s own engines. Engineer John Southern devised a system in which a steam-driven vacuum pump partially evacuated a huge pipe, known then as the “spirit pipe.” Individual coin presses were powered by cylinders and pistons connected to the spirit pipe.

Since the time of Watt and Southern, vacuum power distribution has occasionally resurfaced in different places. Vintage automobiles from the 1920s on were sometimes fitted with a kind of vacuum engine to operate the windshield wipers, using the vacuum from the gasoline engine’s inlet manifold. It cannot have been an ideal system: if an engine turns over slowly, the vacuum from the engine would decrease and the wipers would operate more slowly. If you were driving one of these old cars and saw an approaching hazard, you would naturally slow down. And just when you needed more wipes of the windshield to see what was going on, the opposite would happen: the wipers would slow down and you would be left peering through rain-swept glass at exactly the wrong moment!

Today this principle is still being used in at least one application (albeit rarely): vacuum cleaners. In some models of cylinder vacuum cleaners with a rotating brush, the brush is powered by a simple turbine device that is turned by air sucked into a vacuum created by a centrifugal fan in the cylinder.

Our vacuum engine is a “steam engine” type of device. Unlike most steam engines, however, it does not require a fully equipped workshop with lathe, milling machine, and so on. Neither does it need the thousandth-of-an-inch accuracy required of a working model steam engine. The vacuum engine only requires a few hand tools, pieces of wood, plastic tubing, and easily obtained metal hardware, and you don’t need to make anything more accurately than within a millimeter. You won’t burn your fingers, either—because you don’t need steam! It is also easy to make—you can probably assemble one in an afternoon. Nevertheless, it well illustrates all the main working principles of steam engines: piston and cylinder, crank, flywheel, valve gear, and valve timing. Take a look at books like that of Semmens and Goldfinch if you want to know more about steam engines.

What you need
  • Vacuum cleaner (ideally the horizontal cylinder kind)
  • Short section of 18-mm (3/4-inch) hose
  • 300-mm-long, 32-mm-diameter plastic pipe
  • ca. 150-mm-long, 31-mm-diameter round section of wood to fit snugly in pipe
  • Flywheel pulley from an old washing machine
  • Brass rod that will roughly fit the hole in the flywheel
  • Metal shaft and brackets
  • Conrods (e.g., 8-inch by 1/2-inch Erector set strips)
  • Wood pieces
  • Electric drill
  • Bolts and nuts
  • Hot-melt glue

How to build
The basic idea of the vacuum engine is that a piston is propelled up and down to push a crank that connects to a flywheel. The piston is activated by atmospheric pressure on its connecting rod (conrod) side, with periodic pulses of vacuum applied to its piston-head side. The pulses of vacuum pressure are applied by intermittently connecting the low pressure from a vacuum cleaner to the piston. The intermittent connection is made by a slide valve. The valve is synchronized to the flywheel rotation and hence to the piston movement, by being actuated 90 degrees out of phase with the piston in terms of flywheel position.

I found a piece of wooden dowel that fit snugly inside the drainpipe I had chosen. I then used this rod and pipe for both the piston and the slide valve. I suggest that you aim for a piston that is about 1 mm smaller in diameter than the cylinder, both for the piston and for the slide-valve assembly. Try to find plastic pipe that is close to precisely round. (You will find occasional pipes or sometimes even entire batches that are appreciably noncircular; perhaps they have been squashed in storage or loading at the factory or supplier.)

The piston, if it is the right size, needs no preparation at all other than to bevel the edges and to screw on the conrod bracket. The slide valve and its cylinder are more complicated. The cylinder needs two or three holes (an air inlet hole is optional) as shown in the diagram, which all need their edges smoothed. You must drill through the valve body for the vacuum port and then make a slot with a chisel for the transfer port. The transfer port allows air into the drive cylinder after it has completed its power stroke.

I have two suggestions for alternative, simpler slide-valve designs: First, you can omit the air-inlet hole and the transfer port channel, relying on air leaking around the piston and valve. Second, you can omit the air hole and transfer port from the valve body and also cap its end. You can now switch the valve on and off with a simple cylindrical piston (exactly like the power piston), by arranging that the piston just uncovers the holes in the valve body as it reaches top dead center (TDC). You will need to cap the end of the valve cylinder in this design too.

I used Erector set parts to construct the crank plate and light steel strips for the conrods. The pipe work was completed with a washing-machine drain hose, which is typically a fairly generous-bore 18-mm (3/4-inch) corrugated pipe. You can minimize the Erector set parts by making your own bearing for the flywheel and fitting the crank pin directly into a small hole drilled into the flywheel. The bearing for the flywheel can be made using a piece of 6-mm (1/4-inch) steel and a piece of brass rod around 15 mm (5/8 inch) in diameter, glued with epoxy adhesive into the center of the flywheel central hole. Bore out the middle of the brass rod with a 6-mm (1/4-inch) drill, deburr it if necessary with an oversize drill bit or just a sharp knife, then run the drill up and down it a few times until the rod will fit snugly but freely rotate around the 6-mm (1/4-inch) steel rod.

The position of the cylinder on the base plate is not critical. The position of the valve body, however, is more sensitive: it must just begin to open to vacuum when the piston is closest to the flywheel (the position conventionally known as TDC).

You must ensure that every part can move freely. Check that the edges of the holes in the valve piston cylinder are smooth and that the pivots on the pistons and the crank are not binding. If rotated vigorously by hand without the vacuum applied, the engine should turn over at least three or four times. If you find that the engine slows more quickly than this, you should check for excess friction in one of the parts.

What you do
Without a plentiful supply of vacuum, your engine won’t work, so make sure that your vacuum cleaner has powerful suction. The stronger the vacuum—meaning the larger the negative pressure relative to atmosphere—the better the vacuum engine will perform. If you hold any doubts concerning the performance of the vacuum cleaner, try to find some means of measuring the negative pressure it produces. The flow rate that the vacuum cleaner can produce is rather less important, as the flow rate needed by the vacuum engine is fairly low and, unless your fabrication of the device is more precise than I have suggested, much of the air flow will go to supplying leaks rather than to propelling the engine. If your vacuum cleaner has a low flow rate, you can still operate a vacuum engine, but you must make the piston and valve pieces a tighter fit within their cylinders.

Now position the flywheel just a little past the TDC. Apply the vacuum. With luck, you should find that the flywheel should begin to turn of its own accord, rushing down toward bottom dead center (BDC) and then beginning to slow down. But it should be going just fast enough to rotate one complete revolution at low speed, after which the process can repeat. The next time the engine will reach TDC a little faster, and the flywheel will complete its revolution more quickly. With the dimensions given here and a reasonably powerful vacuum cleaner, your vacuum engine should build up in speed until it is whirling around at 300 to 400 rpm or more.

How it works
The vacuum engine works by atmospheric air pressure. When the flywheel is at TDC, air pressure is the same on both sides of the piston, so no force is applied. With the flywheel turned a little, so that the valve opens to the vacuum, air is removed from underneath the piston. With no air pressure below but atmospheric air pressure above, the piston is forced downward.

Curiously, in the engines I have tried, the rather rough-and-ready fit of the wooden piston to cylinder may help, in that the air inlet and the transfer passage in the valve gear did not seem to be necessary. As mentioned earlier, this means that you can simplify the engine and use a piston as the slide valve. With a better standard of construction, you will need a proper slide valve with an air inlet.

Like the original steam engines, your vacuum engine may need a little adjustment before it will run properly (or perhaps run at all). You do need to ensure that all parts run smoothly and are lubricated with a little light oil such as bicycle oil.

The highest friction forces, assuming that all your components are smooth running under freewheel conditions, will be developed when the vacuum is applied to the slide valve. With a fairly close-fitting slide valve, this force will be reasonably low. If, however, like me, you started with a rather loose-fitting slide valve, you will find that it tends to bind. What is happening here is that the valve piston is being pulled hard against the valve cylinder because of air pressure on the side opposite the vacuum cleaner connection. Some oil may fix the problem. If the fit is really loose, worse than 1.5 mm smaller than the cylinder, then it may be necessary to start again and make another slide-valve piston with a better fit. A simpler solution if the fit is not too bad is to glue a cap onto the end of the slide-valve cylinder. This blocks half the flow of leakage air to the slide valve and reduces the force needed to operate the valve. (Thanks to the kids at the Saturday Activity Center in Guildford, U.K., for that tip.) Of course, if you have used the simplified piston-style slide valve, then you will have blocked off the end anyway.

How to build simple motor stepper

The stepper motor is the rather complex type of motor at the heart of much modern equipment. Computers, printers, CDs, DVDs, photocopiers, and countless other machines right up to industrial robots all rely upon it. The stepper motor is designed to respond digitally, which of course makes an excellent match with computers. Send a command to move 713 steps to the interface circuit, and that circuit will pulse the several sets of coils on and off 713 times until the motor output shaft has carried out the instruction.

The stepper might superficially resemble an ordinary motor—it is just coils, iron, and magnets, after all. It differs, though, in several ways. First, it is specifically designed to be able hold a stationary position. Most ordinary motors can’t remain stationary; this project relies on a particularly simple kind of ordinary motor that does happen to have some position-holding ability. Second, the stepper motor is not, like an ordinary motor, designed to run continuously when you supply current; instead it is intended to move one step and then stop. Last, unlike ordinary motors, which connect directly to an ordinary electricity supply like a battery, stepper motors are useless without their matching driver circuit. (The sidebar for this project describes what that driver circuit does.)

In the early days of power electronics, stepper motors were expensive devices, and very large ones still are. Mass production has brought the price of small steppers down to only $10 or $20, which is why they are so widespread, even in simple computer printers costing only $70. Happily, we can demonstrate all the principles of a stepper motor and all its complex sets of coils and driving circuits with just a simple capacitor and a simple 50-cent motor.

What you need

  • A small (10 mm × 10 mm × 20 mm) two-magnet, three-coil DC motor, such as the kind used in countless motorized toys
  • Lightweight plastic wheel to fit on end of motor
  • Changeover switch, ideally a microswitch, 5A current rating
  • Capacitor
  • Battery
  • Wires

For the AC Jiggler

  • Transformer (e.g., 12-V AC output)
  • Resistor (correct value to be selected, but probably from 200 to 2000 ohms)

What you do

Suitable motors should be available from stores that supply small electronic parts, or you can order one from an educational supplier for as little as 30 cents. A simple motor of the type we need has two magnets and three iron-cored coils on its armature or rotor (the rotating part). You should find that when you rotate it, it tends to stop in one of six positions. It should just click into place, almost as if it had six mechanical detent stops. If you fit a wheel onto the motor shaft and mark a line on the wheel, you should be able to check this out precisely. The motor does not have any actual mechanical detents; this effect is due to one of the iron cores lining up with one of the magnets, a phenomenon described in my book Ink Sandwiches, Electric Worms (Experiment 21, “Motor Dice”). The most common motor of this type has a cylinder shape with two opposing flats around the circular cross section, typically about 15 mm across the flats, 19 mm in diameter.

If you can’t easily buy such a motor, you can take apart a few discarded motorized toys until you find one. There are other motors in which the spacing and small number of magnets and cores in the armature mean that they also have positive location angles around the axis. The small motor I recommend, however, has only six stable points; because these six points are basically equivalent in the motors I have, they are particularly suitable.

First you must set up the stepper motor circuit: the motor is arranged to be driven from the capacitor, not from the battery. Each time you operate the switch up and then down, the motor should jump to its next stable position. With each cycle of the switch up and down, the capacitor is first charged up, then discharged through the motor, causing it to hop along by one pole. I used pulses of 6 V from a 2,000 microfarad (µF) capacitor. With these values, I found that I could step six times per revolution, with a rate of 2 Hz. What seems to matter is the energy in the pulse provided. You could use a smaller capacitor, for example 470 µF, but charge it up to a larger voltage of about 12 V.

You can use a simple toggle changeover switch, but it will limit the speed of operation. If you can find a changeover microswitch or a push-button changeover switch, you can run the motor at higher stepping speeds.

Occasionally the 50-cent stepper will jam in a position between its stable positions. This situation becomes clearer if you put a small, lightweight wheel on the motor shaft, as suggested, with an index mark—a large arrow or something —so that you can clearly see whether the motor is stopping between positions. Friction from the commutator is probably mainly responsible for a motor’s stopping between stable positions, but the externally connected load and any gearwheels connecting the motor to that load may also have some effect.

A separate “jiggler” current supply can be connected to deal with a sticking problem as follows: Use an AC transformer connected to the domestic electricity to supply low voltage AC. The AC low-voltage value can be anything from 3 to 30 V, since you should connect it via a large resistor to the motor. Choose the resistor value to yield a suitably low jiggler current: try a 500 or 1,000 ohm resistor to start with, or otherwise select a resistor that yields a current of around 10 mA.

How it works

If you have access to an oscilloscope, you can see what is happening. Connect the motor to a battery and then connect the oscilloscope to it: you will see sharp, high spikes and longer, lower lumps in the voltage as the motor draws and then stops drawing current while the commutator rotates. Put a low resistance like 1 ohm in the motor lead (use a high-current resistor like a 1-W or 10-W type so you don’t burn the resistor out), and then you can precisely measure the current flowing. You should find sharp pulses coming from the capacitor and less spectacular spikes coming from the motor: we calculate below how wide the capacitor pulses should be, but something on the order of 5–20 milliseconds is probably what you will get with the recommended values below.

The sharp, high voltage spikes derive from the motor’s armature coil and its iron core. The motor coil stores small amounts of energy in the magnetic field in the iron core, and this energy is released in sharp spikes of voltage when that field collapses (i.e., when the motor is disconnected by the commutator). This is why electric motors are noisy, electrically speaking, and radiate radio-wave noise that can be picked up on radio and television sets. Most commercial commutator motors, like the ones in your vacuum cleaner or electric drill, use capacitors to suppress this effect.