THE ENGLISH PHILOSOPHER JOSEPH PRIESTLEY remarked, writing to Benjamin Franklin in 1770, that "a good electrometer is one of the greatest desiderata among practical electricians to measure both the precise degree of the electrification of any body and also the exact quantity of a charge." Practical electricians still have reason to applaud Priestley's enthusiasm for the electrometer even though the instrument has been displaced in most laboratory uses by cathode-ray oscilloscopes. The electrometer remains the preferred instrument for measuring minute charges such as those induced in small insulated objects by potential differences in the atmosphere, by one in other and by the disintegration of radioactive substances.

The electrometer Priestley described in his letter to Franklin had been built by a Mr. Henley of London. It consisted of "an exceedingly light rod, with a cork ball at the extremity." The rod was hinged to a somewhat thicker rod of ivory When a charge was placed on the device, the cork ball and the thick rod repelled each other, the light rod swinging outward at an angle that depended on the strength of the charge. In this form the device was merely an electroscope. Henley converted it into an electrometer by equipping the thick rod with a semicircular scale that, in measuring the angle at which the light rod was repelled, also showed the magnitude of the charge.

The sensitivity of the best modern electrometers far surpasses that of Henley's instrument. The modern instruments also cost more. Most of them have a special vacuum tube for sensing the charge under measurement and are priced at several hundred dollars. John L. Menke of Barnesville, Md., recently constructed an inexpensive electrometer that anyone can build at home using ordinary vacuum tubes.

Menke's instrument employs a vibrating capacitor that in effect transforms the electrostatic charge under measurement into an alternating current that can be amplified and subsequently changed back into direct current for operating a conventional milliammeter. It easily measures direct current as small as a tenth of a trillionth of an ampere- less than a billionth of the current drawn by the smallest flashlight bulb. Menke writes: "One can determine potential differences by measuring the force of attraction between the plates of a capacitor that carries the unknown charge. The only current drawn by such an instrument is the current that leaks through and over the surfaces of the insulating materials that support the plates of the capacitor. In effect the device consists of a mechanical balance for weighing the unknown charge. It is not only fragile but also insensitive to potentials of less than about 100 volts.

"Instruments of the type I constructed are known as vibrating-reed electrometers. The essential element in the instrument is a capacitor with one fixed plate. The second plate is vibrated by an electromagnet. The amount of charge the plates can share at a constant voltage changes as the spacing between the plates varies. The voltage, V, that appears across the plates of a capacitor is equal to the ratio of the charge, Q, to the capacitance, C. (V = Q/C.)

"When the spacing between the plates is changed, as by vibrating one of the plates, the voltage varies in step with the vibration. A charge placed on the vibrating capacitor then appears as an alternating voltage. The capacitor must be well insulated, of course, or the charge will leak off. The insulation resistance of good capacitors usually amounts to at least a billion ohms. Several kinds of plastic are readily available for constructing capacitors of this resistance, although surface leakage may limit their performance on days of high humidity.

"For one plate of the variable capacitor of my electrometer I used the diaphragm of an ordinary magnetic earphone. The diaphragm is vibrated by connecting the solenoid of the earphone to the secondary winding of a transformer designed to heat the filament of a vacuum tube. The transformer is powered by 60-cycle house current. The second plate of the capacitor consists of a disk of flat sheet metal about the size of a 25-cent piece. The disk is soldered at the center to the end of a machine screw that fits a threaded hole in a bridge, which is flexibly attached to the case of the earphone [Figure 1]. The bridge is made of transparent sheet plastic.

"The screws that attach the bridge to the case of the earphone serve as adjustments for altering the plane of the disk with respect to that of the diaphragm. The screw to which the disk is attached serves as an adjustment for altering the spacing between the plates of the capacitor, as shown in the accompanying illustration [Figure 2 ]. The spacing between the plates must be made as small as possible so that the variation in the capacity, and hence the change in voltage, will be as large as possible. Sharp edges on a conductor as well as burrs and dust encourage the leakage of charge and must be eliminated. To insulate the input lead that connects the fixed plate to the charge under measurement I used a conventional Teflon connector of the type designed for ultra-high-frequency radio apparatus. An unsupported wire would be better electrically but less convenient.

"The alternating voltage that appears across the vibrating capacitor ["C" in illustration on the left] is coupled to the input capacitor of an amplifier of the cathode-follower type. The insulation resistance ["C'" in the illustration] of the input capacitor must be at least 1,000 megohms-a billion ohms. I used an inexpensive ceramic capacitor, rated at five trillionths of a farad (five picofarads). The capacitor is insulated for a potential of 1,000 volts.

"Several capacitors of other types were tried in the circuit, including one insulated with high-quality mica. The ceramic unit finally selected was as good as any, although individual variation among the ceramic capacitors I have is enormous. Doubtless a homemade capacitor improvised of 1/2-inch square plates spaced 1/16 inch apart in air would be effective. The input capacitance of the assembled instrument is 13 picofarads. During operation the vibrating capacitor varies by about three picofarads.

"The signal can be taken directly from the first triode section of the vacuum tube or amplified further by the second section of the tube [Figure 4]. Either an oscilloscope or a high-impedance alternating-current voltmeter can be used for readout. It is also possible to include a vacuum-tube voltmeter circuit in the instrument and to employ a conventional milliammeter for readout.

"The use of an oscilloscope simplifies the adjustment of the vibrating capacitor, because even the slightest change in the physical position of the plates shows unmistakably in the wave form of the signal. The circuit includes an adjustment for minimizing 60-cycle hum, which is always present in the signal. The oscilloscope is also useful for monitoring this adjustment, since the hum is easily distinguished from the signal by the difference in phase of the two wave forms.

"A watch with a radium dial turned out to be a good source of charge for testing the instrument. The crystal of the watch was removed. Radium emits a variety of radiation during its chain of decay, including both beta particles (electrons) and alpha particles. The positively charged alpha particles outnumber the negatively charged beta particles. Hence a net positive charge leaves the watch, and the case acquires a net negative charge. The negative voltage increases until leakage to ground and through the air balances the charge leaving the watch. I simply hang the watch on the input terminal with an alligator clip and monitor the output voltage. The adjustment screws of the vibrating capacitor are then manipulated to produce maximum output. The balance control is again adjusted for minimum hum.

"An experimenter who constructs the instrument will be impressed by the amount of charge picked up by his body. A finger touching the input terminal produces an enormous signal that requires some time to die away. The effect is caused by alternating current induced in the body by the house wiring, by electrostatic charge generate by the friction of one's clothing an by the soles of the shoes scuffing the floor. Stray pickup from these source can be minimized by enclosing the circuit assembly in a sealed metal box. Adjustments can then be made by inserting an insulated screwdriver through holes in the box. The operator should wear static-free clothing, such as cotton. He should also sit on a metal stool and wear a grounded metal watchband or bracelet. The instrument easily detects the voltage generated by pulling a cloth across a table 10 feet away!

"Maximum sensitivity can be achieved and maintained by keeping scrupulously clean all insulating surfaces associated with the circuits of the vibrating capacitor and input capacitor and by sealing a desiccant inside the metal housing. Clean these surfaces with acetone, alcohol and distilled water in that order. Repeat this procedure twice and let the parts drip dry-without wiping. Thereafter do not touch or wipe the surfaces, otherwise conducting paths will be created that will reduce the sensitivity.

"For calibrating the instrument I had access to a source of beta rays that emitted 1,000 particles per second, producing a one-volt signal in the output. The average energy of the emitted beta particles was three million electron volts. About 100,000 charge pairs were therefore produced in the air before the average particle came to rest, because the ionization potential of air is about 30 electron volts. Since a voltage appears on the input terminal many of these charges as well as other charges naturally present in the air are collected by the terminal. The instrument is therefore sensitive to a current of about a tenth of a trillionth of an ampere at an inferred input resistance of 60 billion ohms.

"Vibrating-reed electrometers are used routinely by nuclear physicists for measuring the accumulated charge (voltage) on the plates of ionization chambers. A primitive ionization chamber can be assembled at home from a pair of insulated tin cans-a small one such as a frozen-fruit-juice contained nested concentrically inside a standard No. 303 can. Circuit details are described in standard reference texts. Another application for the instrument is suggested by the precautions the experimenter must take to prevent stray pickup from influencing the output: the vibrating-reed electrometer makes a dandy burglar alarm!

"Several improvements are possible. For example, the hum can be reduced by heating the filament of the tube by direct current. A further improvement could be made by driving the earphone at some frequency other than 60 cycles. An amplifier tuned to this higher or lower frequency would reject the 60-cycle currents induced in the input circuit by the power line and so would in effect increase the sensitivity. Heavy shielding around the earphone, and in particular around the coil would diminish the noise induced in the input by the current that drives the earphone. The input resistance of the unit could be increased by using a two-stage amplifier of the cathode-follower type to increase the signal level. Finally, the sign of the charge under measurement must now be inferred by observing the phase of the output signal in relation to that of the current that drives the earphone. A phase-sensitive circuit could be added for displaying this information automatically."

Many experiments require the accurate measurement of minute electric currents, for which the vibrating-reed electrometer is well suited. Most such experiments also require the measurement and close control of temperature. The measurement of temperature, which is analogous to voltage, poses no problem because accurate and inexpensive thermometers are readily available. The close, automatic regulation of temperature, however, has always posed a problem for amateurs. Inexpensive thermostats of the necessary accuracy are difficult to find in most communities, and the use of thermistors involves circuit constructions that are difficult to calibrate over extended temperature ranges.

Carl Henry of Chattanooga, Tenn., has solved this problem by designing a temperature-control apparatus around an ordinary thermometer. Instead of reading the thermometer by eye and adjusting an electric heating element by hand he uses one of the new cadmium selenide photocells to monitor the thermometer and to control the heating current automatically by actuating a relay in the heating circuit. Henry writes: "Depending on-the quality of the thermometer, the unit can automatically control temperature within one degree. The apparatus can be built for less than $10. That price makes it, if nothing else, the most inexpensive temperature regulator now available.

"The scheme of operation is based on the interruption of a light beam by the column of mercury in a tubular thermometer. The intensity of the beam controls the output current of a cadmium selenide photocell. The operating characteristics of these new semiconducting cells differ widely from those of conventional cells with which amateur experimenters are familiar. The electrical resistance, for example, changes by a factor of a million when the incident light varies from only one to 100 foot-candles. Moreover, the sensitivity to light of low intensity is impressively greater than that of conventional cells. Most of the problems encountered during attempts to use the cells in new applications arise from excessive light and stray light.

"My temperature-sensing unit consists of a miniature incandescent lamp and a lens that directs a beam of light transversely through the thermometer to the photocell [Figure 5]. The lamp, lens, thermometer and photocell are enclosed in a light-proof housing through which the thermometer slides. The position of the thermometer with respect to the housing determines the operating temperature. A small rectifier powered by 60-cycle alternating current supplies filtered direct current to the relay through the photocell, energizing both the relay and the photocell. The lamp can be energized by a transformer that operates from the power line if the voltage of the line is reasonably stable. Otherwise the lamp should be operated from a dry battery, because the photocell does not distinguish between fluctuating temperature and varying supply voltage.

"When the column of mercury does not interrupt the light beam, the photocell receives maximum illumination. Its resistance is then typically about 100 ohms. The current at this maximum illumination is about 20 milliamperes, sufficient to operate a sensitive relay. The contacts of my relay are large enough to switch the current of a 100-watt heating element. If I wanted to control a larger heater, I would insert a heavy relay in the heater circuit and operate it by the sensitive relay.

"More than one photocell must be used for controlling temperature within narrow limits. The rising column of mercury in the thermometer reduces the light to the photocell, causing the relay to open and switch off the heating element. The heated element continues to radiate for a time, however, depending on its size. As a result the temperature may rise above the level wanted. Similarly, when the mercury column drops and the relay closes, some time is required for the heating element to warm up. During this interval the temperature may drop below the desired level. At temperatures above 150 degrees Fahrenheit such overshooting is not troublesome.

"Close control at lower temperatures is achieved by equipping the thermometer with two or more photocells spaced about five degrees apart. Each photocell actuates a companion relay. The relays operate in sequence as the height of the mercury column changes and can be connected to individual heaters or wired in such a way that they sequentially short-circuit appropriate resistors that are in series with the heating element. The closest control can be achieved by using a thermometer of the expanded-scale type."

Another versatile semiconducting device, which has been largely overlooked by amateur experimenters, is the tunnel diode [see "The Amateur Scientist," March, 1963]. In a sense the function of the tunnel diode is opposite to that of the semiconducting rectifier: the diode can transform direct current into alternating current. Harry E. Stockman, professor of electrical engineering at the Lowell Technological Institute in Lowell, Mass., recently used a tunnel diode to build what is perhaps the world's simplest miniature motor-one that requires neither a commutator nor brushes. The device, which he has patented, has only three essential parts: the tunnel diode, a small coil and a bar magnet that turns on a pivoted shaft [see Figure 7].

The motor must be started by hand. Once it is started, current induced in the coil by the magnetic field of the bar magnet causes the tunnel diode to function as an oscillator. The diode s then energizes the coil with alternating s current. The alternating magnetic field generated by the coil interacts with the r permanent field of the magnet, causing it to rotate in synchronism with the oscillations of the tunnel diode.

The dimensions of the magnet are not critical. Alnico bar magnets about a quarter of an inch square and two inches long work well. Alnico is not easy to drill. The pivoted shaft can be cut in two, however, and the ends cemented to the magnet with epoxy resin. The coil can be wound on a wooden form of sufficient size to provide an air space for the magnet.

A pen-cell flashlight battery will drive the motor for many hours, because the unit draws only a few thousandths of a watt. I made one version of the motor that was equipped with glass pivots. It has been operating continuously on a solar battery for a year and should continue indefinitely. Incidentally, by attaching one end of the bar magnet to the shaft and suspending it vertically the magnet becomes a pendulum. Electrical noise naturally present in the circuit starts the pendulum swinging. A few millionths of a watt will keep it going. I made a pendulum that operated from a battery improvised with blotting paper sandwiched between a nickel and a penny. When the leads of the coil were attached to the coins and the paper was moistened with saliva, the pendulum promptly went into action.

Bibliography

RADIOISOTOPE LABORATORY TECHNIQUES. R. A. Faires and B. H. Parks. Pitman Publishing Corporation, 1958.

TEMPERATURE MEASUREMENT IN ENGINEERING. H. Dean Baker, E. A. Ryder and N. H. Baker. John Wiley & Sons, Inc., 1953.

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