A SCHEME whereby an inexpensive radio can be modified with a few accessories and put to work in experiments that fascinate children while leading them to an understanding of various natural phenomena has been devised by Adrian Popa of Newbury Park, Calif. He undertook the venture with his sons Brian and Mark, aged 12 and eight respectively. The radio cost about $4 and the outlay for accessories was trifling. Popa writes:

"Our project began when my sons asked how fast a hummingbird beats its wings when it hovers at our feeder. It occurred to me that perhaps an experiment could be devised that would enable the boys to time the wingbeats themselves. My goal was to keep the experiment and the related construction work at the level of youngsters while making the enterprise complex enough to stimulate original thinking.

"The transistor radio that was the principal instrument for the experiments is slightly larger than a pack of cigarettes. Sets of this kind include a sensitive amplifier that can boost almost any external signal. The amplified signals are transferred into sound by the self-contained loudspeaker.

"Signals from the external source can be fed into the amplifier by a pair of wires connected to the two outermost terminals of the three on the potentiometer that functions as the volume control of the set. (The volume control of some sets also serves to turn the set on and off. In this case the control has five terminals. The first and fifth terminals do the switching. Connect the wires to the second and fourth terminals.) The result of this simple modification is a portable amplifier with a self-contained power supply. The modification does not damage the radio. It can still be tuned to broadcast stations, which my boys often do during long experiments.

"External signals are amplified by first tuning the dial to a point at which no broadcast is heard. The additional wires are then connected to the signal source. We found that the simplest way to bring the wires out of the set was to thread them through the hole into which one normally would plug an earphone. We also learned that the set will not pick up objectionable noise if the wires are twisted together about two turns per inch. Noise can also be reduced by using shielded wire, but the additional reduction is not worth the effort. We install a pair of small alligator clips on the ends of the wires for convenience in connecting the ampler to various signal sources.

"Our bird detector is a photovoltaic cell of the silicon type, commonly known as a solar battery. Selenium and cadmium photocells are more sensitive, but they are of the photoresistive type and require an external source of power. Their use would complicate the circuit.

"Cells of the solar-battery type are more sensitive than needed for these experiments. Indeed, they tend to become saturated in bright sunlight. We prevent saturation by covering the cell with a mask containing a small hole, which cuts off most of the incident light.

"We tested the apparatus by clipping the amplifier to the cell and exposing the cell to a fluorescent light. The loudspeaker emitted a hum at a pitch of 120 vibrations per second when the light was on. That is the characteristic rate at which these lamps flicker. The hum stopped when we covered the cell.

"What we had now was an optical receiver. We placed it on the floor near a window of our living room where a shadow is cast by hummingbirds that feed outside [see illustration, left]. We detected many interesting sounds that are made by the movements of the birds in addition to the low-pitched hum generated by their flapping wings. The apparatus also served as an excellent alarm that sounded when the birds fed. It enabled us to do other things while waiting for them to arrive. We have enjoyed many hours of bird watching without having to keep our eyes constantly glued on the feeder.

"Now that we had an optical receiver for detecting the movement of a bird's wings, we needed a clock to time the rate of the wingbeat. We decided to compare the rate with a tone of known pitch by using our ear, just as a piano tuner compares the tone of a piano string to that of a tuning fork. We knew we could generate a flickering light by punching spaced holes near the edge of a disk of cardboard and rotating the holes in front of a flashlight. We could convert the flickers into sound with the optical receiver. Moreover, we could learn the frequency at which the light flickered by multiplying the number of holes in the disk by the rate, in revolutions per second, at which the disk turns.

"We used a cardboard disk 14 inches in diameter. Several sets of holes were punched in the edge [see top illustration on next page]. We rotated the disk at known rates with a phonograph turntable. The wing rate was determined by shining a flashlight through the holes to the photocell at the same time that the bird signal was present.

"The boys observed that the wingbeat of a hovering bird was close to 41 vibrations per second, as generated by the perforated disk. The rate of 37.5 vibrations per second was a bit low and the 45-cycle rate was high. We were all astonished to learn that the hovering rate of big and little hummingbirds is the same. As the birds accelerated to leave the feeder we observed rates as high as 80 wingbeats per second, the value that is listed in our encyclopedia.

"The birds were not always cooperative. Often they would not hover long enough for us to make a good measurement. We solved this problem with a small tape recorder. We could replay the recorded sounds at leisure as many times as necessary to make an accurate comparison with the frequencies of our standard disk. We are now planning a series of experiments to measure the wing rates of insects. These experiments will require a more complex optical system, which we are now building.

"Our flashlight modulator, in the form of the perforated cardboard disk, led to a number of other experiments. We found, for example, that a series of interesting musical effects can be generated by punching evenly spaced long, short and odd-shaped holes in the edge of the disk. In another experiment we mounted a smaller perforated disk on the shaft of a toy motor. With this motorized light-chopper we could detect the modulated flashlight beam at a distance of several hundred feet. We used the system to send signals in Morse code. At night, by substituting the headlight of an automobile for the flashlight, we could detect the modulated beam at a distance of several hundred yards.

"We also modified the experiment to simulate the direction-finding apparatus of an airport instrument-landing system. Two sets of holes were punched side by side about an inch apart in a disk to generate simultaneously 15 and 30 cycles per second. The perforations were spaced so that one series of holes passed through one half of the flashlight beam and the other set passed through the other half [see illustration, left].

"When the optical receiver is held in the beam at either side, one tone is louder than the other. Both tones are equally loud when the photocell is held exactly in the center of the beam. Blindfolded children and adults have used the system to find their way along the beam directly to the flashlight, just as airplane pilots follow an audio-modulated radio beam to the center line of a runway. After this demonstration even schoolchildren in the second grade can understand the concept of an all-weather instrument-landing system.

"We next altered the system to demonstrate the transmission of sound over a beam of light. We made a voice modulator with a one-pound coffee can plus a few other odds and ends that we found at home [see illustration at right]. We cut the bottom out of the can and replaced it with a sheet of plastic kitchen wrap held in place by a rubber band. On the side of the can we soldered a quarter-inch nut with 20 threads per inch. The nut enabled us to mount the can on a camera tripod.

"Sound waves that enter the open end of the can cause the plastic diaphragm to vibrate. We reflected the beam of the flashlight off the diaphragm to our optical receiver. By talking into the can we could transmit speech of good fidelity up to about 50 feet.

"The plastic diaphragm turned out to be a poor reflector. Most of the light passed through the clear plastic. We tried a diaphragm of aluminum foil. The transmission range increased substantially but the fidelity was poor. I obtained Mylar film .00025 inch thick that was coated on one side with a film of aluminum. Both the quality of the reproduced sound and the range of this modulator were excellent.

"The modulator picked up voices 20 feet from the can and could transmit them several hundred feet on the light beam. It also made a fascinating passive modulator for 'spy' use, which the boys easily related to programs they had seen on television. I have substituted a laser beam for the flashlight in both the simulated system and the voice modulator for demonstrations at schools and colleges to illustrate my field of laser communications.

"The transistor radio can of course amplify signals from transducers other than photocells. For example, an inexpensive crystal microphone can be connected to the amplifier by a long shielded cable for monitoring the sounds made by a baby in a nursery. An intercommunicating system can be made with a pair of such microphones and radios. A small coil of wire, such as a one-millihenry radio-frequency choke coil, picks up a substantial signal when it is placed near the receiver of a telephone. When the coil is connected to the input of the transistor amplifier, both sides of the telephone conversation are clearly reproduced.

"The amplifier can be converted into a tone generator by connecting one terminal of a capacitor to one terminal of the loudspeaker or to one terminal of the earphone jack and the other terminal of the capacitor to one terminal of the volume control [see illustration, left]. The capacitor feeds energy from the output of the amplifier back to its input-an action that generates continuous oscillations. The unit oscillates best when the capacitor is connected to a selected terminal of the potentiometer. Try both terminals of the potentiometer and make the connection to the one that works best. The capacitor can be of any value between 100 picofarads and one microfarad.

"The pitch of the tone varies inversely with the size of the capacitor. The pitch can also be varied through an appreciable range by adjusting the volume control. The oscillator can be used to generate an alarm signal. We have also employed it for practicing Morse code. The tone can be turned on and off by inserting a switch or a telegraph key in series with the capacitor.

"Another series of experiments that interested us involves the detection of military and airport radar stations. We pick up the radar signals by an antenna made of a paper clip [see top illustration on next page]. The signals are detected with a microwave diode of either Type lN21 or Type 1N23. Both diodes can detect all the radar bands in common use, but the lN21 is best for signals of 10-centimeter wavelength, called the S band, and the 1N23 is best for the three-centimeter wavelength, or X band.

"The diodes detect the sweep rate (the rate at which the beam of the radar scans the area) and the rate at which the beam pulses. Typically radar sweeps an area about twice per minute and the beam pulses about 400 times per second, a frequency approximately equivalent on the musical scale to G above middle C. The paper clip functions as an omnidirectional antenna: it picks up signals almost equally well from all directions. We have detected airport and Nike missile radars up to 10 miles away.

"The apparatus can be employed to demonstrate the principles on which highly directional antennas work. For example, the direction of a radar station from the experimenter can be established by putting a corner reflector on the paper-clip antenna. Cement aluminum foil to one side of a rectangular sheet of cardboard four inches wide and 12 inches long. Fold the strip to make a right angle. Support the paper-clip antenna inside the comer parallel to the fold. Brackets to support the antenna at this position can be made of cardboard.

"The strength of the received signal will be at a maximum when the distance between the antenna and the corner fold is equal either to half of the length of the incoming radio wave or to one and a half wavelengths. The signal strength will approach zero when the separation is one wavelength. In the case of 10-centimeter waves the signal will be heard at maximum volume when the antenna is either two or six inches from the reflector. The signal will be heard loudest when the reflector is pointed directly toward the radar station.

"In a similar way one can determine with this apparatus the location of large metal objects that reflect radar waves, such as aircraft and water tanks. Of course, the set is not nearly so sensitive as commercial receivers that are designed to pick up radar signals, but it clearly demonstrates the optical nature of radio waves. The fact that radio beams are a function of antenna geometry suggests other experiments, such as making dish-shaped reflectors (like those of optical and radio telescopes) and miniature versions of conventional television antennas.

"An antenna of an entirely different type can be used for picking up the eerie signals, generated by lightning, that bounce back and forth between the Northern and Southern hemispheres of the earth. When these signals are transformed into sound, they are musical, but they differ in character according to the circumstances of their origin. Some are mere clicks. The click is heard when lightning strikes within 100 miles or so of the observer. If the stroke occurs 1,000 or so miles away, some of the emitted radio waves may reach the observer by bouncing off the ionosphere. They are heard as a short, high-pitched note known as a 'tweek' or a 'chink.'

"If the stroke occurs in the Northern Hemisphere at the middle or higher latitudes, a portion of the energy may be propagated along a line of the earth's magnetic field. The flux line guides the signal in an arching path to an altitude of some 8,000 miles; there it bends downward and returns to the surface in the Southern Hemisphere. Here the energy is reflected, returning along the same arching path to the general area where the stroke occurred.

"In the course of this long excursion the band of frequencies constituting the signal becomes separated in time. The shorter waves travel through the ionized atmosphere at higher velocity than longer waves. The transformed signal is heard by the observer as a piercing tone of falling pitch, known as a 'whistler.' The returned energy may be reflected again and again. In effect, the 'click' generates a series of whistlers, each persisting longer than its predecessor and with greater separation between tones of high and low pitch. The tones of a whistler as reproduced by the amplifier may range in frequency from about 6,000 to 500 vibrations per second.

"To hear these effects (as well as the 'dawn chorus,' an unexplained galaxy of birdlike sounds that appears to accompany magnetic storms) the experimenter must select a location remote from electrical power lines and other sources of man-made electrical noise. Attach to one lead of the amplifier a copper wire about 300 feet long. The wire serves as the antenna. Attach the distant end of the wire through an insulator to the highest available object, so that the antenna slants upward from the amplifier. Connect the other lead of the amplifier to a metal stake driven into moist soil. Also connect a one-millihenry choke coil in series with a one-microfarad capacitor of the Mylar type and attach the combination across the input of the amplifier. Whistlers are heard frequently in the middle and upper latitudes during seasons of maximum thunderstorm activity.

"In our more recent experiments we have moved on to more complex projects such as radio holography and Doppler radars. For these investigations we still use slightly modified transistor radios. We hope that amateurs will join us in the fun of devising still other ways to probe nature with these versatile sets."

A PHYSICIST investigating phenomena mathematically occasionally writes an equation that suggests a novel device of no obvious use. An example of such a device is an electric motor invented recently by Harry E. Stockman of Arlington, Mass. The machine consists essentially of a rotor in the form of a soft bar of iron supported at its middle by a vertical shaft that is free to turn between the poles of an electromagnet [see illustration at left]. The magnetic field is provided by a solenoid that operates on alternating current. When the solenoid is energized by a 430-hertz current, the polarity of the magnet reverses 3,600 times per minute.

If the rotor consisted of a thin strip of hard steel instead of the soft iron bar, and if the strip were given an initial spin at the rate of 3,600 revolutions per minute, the steel rotor would become permanently magnetized, fall into lockstep with the alternating magnetic field and continue to rotate synchronously with the field. The synchronous motors of electric clocks operate on this principle. Stockman's motor runs asynchronously!

When the soft iron rotor is given start, it continues to turn at a speed that varies roughly with the applied voltage. The tips of the rotor experience a strong force of attraction as they approach the poles of the magnet. The force decreases as the rotor moves away from the poles.

The variation of the force is explained by a similar variation in the inductance of the solenoid, reflecting the property of solenoids, which is to oppose electric current. The inductance of the coil is maximum when the poles of the magnet are bridged by the soft iron bar. The current and the magnetic force of attraction decrease.

Conversely, when the iron bar turns away from the poles, the inductance approaches the minimum and the exciting current and the force of attraction increase. In actuality the force of attraction does not become minimum at the instant the bar reaches its middle position between the poles of the magnet. The field cannot change instantly. Accordingly the bar is strongly attracted as its ends swing toward the poles of the magnet and less strongly attracted as they coast away.

Inductance is known as a parameter of electric circuits: a quantity that is normally constant but that can be varied by circumstance. In the case of this device the determining circumstance is the position of the rotating bar of iron. For that reason Stockman refers to his invention as a "parametric" motor.

The core of the electromagnet can be bent from a strip of flat iron 1/8 inch thick and one inch wide, making a flat-bottomed U with upright legs about 2-1/2 inches long. The rotor can be bent from 1/2-inch stock. The ends of the rotor should be bent upward to form legs 1/2 inch long that leave an air gap of about 1/32 inch at each end when the rotor is placed between the poles of the magnet. The dimensions are not crucial.

The solenoid can be made of approximately 2,700 turns of No. 26 enameled copper wire. The assembled electromagnet will have an inductance of about .3 henry. Stockman connects a capacitor of eight microfarads in series with the solenoid and energizes the unit through a variable-voltage transformer. The capacitor is not strictly necessary, but the operation is greatly facilitated by taking advantage of resonance. The motor-will operate on about five volts. The speed is nonsynchronous and roughly proportional to the applied voltage up to about 10 volts. At higher voltage the machine tends to lock in step with the frequency of the power source and to operate as a conventional synchronous motor.

Bibliography

ELECTROMAGNETICS. John D. Kraus. McGraw-Hill Book Company, Inc., 1953.

THE RADIO AMATEUR'S HANDBOOK. Byron Goodman. American Radio Relay League, 1972.

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