What happened? A number of investigators have worked on the puzzle. One of them is S. L. Khanna, professor of physics at York College in York, Pa. He became interested in electrets as a student of D. R. Bhawalkar of the University of Saugar in India. Khanna writes:

"The idea of constructing a device capable of retaining electrostatic charge was not new when Eguchi made his experiment. The possibility had been foreseen a century earlier by Michael Faraday. No one, however, anticipated the remarkable behavior exhibited by Eguchi's electret.

"Electrets are analogous to magnets in some respects, but they also differ in striking ways. All electrets are manmade, whereas magnets are found in nature. When magnets are stored properly, they can last indefinitely, but the lifetime of electrets appears to be limited. If an electret is severed between its poles, each part retains the electrical properties just as the parts of a broken magnet do. The electret is not destroyed even if the surface layer is scraped off, although the new surface is less strongly charged than the original one. This result shows that the electrification is not a surface phenomenon, like frictional electricity, but a volume effect.

"The electret will remain charged for years if it is wrapped in metallic foil, which short-circuits its charged surfaces, much as magnets are protected by placing a bar of iron across their poles. When an electret is stored without a wrapping of foil, the charge decays, but it will reappear if the unit is short-circuited for a time. Electrets can be made of many kinds of dielectric material, including ceramics. Ceramic electrets retain their charge even when stored without a 'keeper' of metallic foil.

"All electrets are destroyed by melting and are sensitive to humidity. When an electret is placed in humid air, it loses charge temporarily, but it regains charge after being dried. The temporary loss of charge is ascribed to the microscopic layer of moisture that forms on the surfaces and acts as an electrostatic shield. I have found that units are permanently damaged by prolonged exposure to humid air.

"In addition to the electrostatic effect, electrets exhibit pyroelectric and piezoelectric effects. The distribution of charge can be altered by the application of heat or pressure to selected areas of the units. It is important to mention that an electret is not a battery. It supports an electrostatic field but cannot perform work.

"After Eguchi described the device, numerous experimenters searched for dielectric substances that exhibit the electret effect. Almost all waxes work. A favorite formula consists of 45 percent carnauba wax, 45 percent water-white rosin and 10 percent white beeswax.

"It has been my experience that carnauba wax works better than most other waxes, but the experimenter should try as many kinds of wax as he can get. Waxes that become hard and brittle at room temperature may crack. To prevent damage of this kind I usually add some beeswax as a softening agent, particularly when making electrets thinner than one millimeter. At the other extreme, one should avoid making electrets of waxes so soft that they sag out of control.

"It is now believed that all solid insulators exhibit the electret effect to some extent. Electrets have been made from plastics such as polystyrene, Plexiglas, nylon and Teflon, from hard rubber (ebonite), naphthalene, sulfur and sugar. I have not tried ice, which might work because molecules of water are highly polarized. Recently it has been shown that stable electrets can be made from inorganic ceramic dielectrics such as the titanates of calcium, magnesium, zinc, strontium and barium. The barium titanates, which appear to retain the strongest charge, represent the closest electrical analogy to magnetic iron

"The performance of an electret is determined by a number of factors including the material of which it is made, the temperature at which it is formed, the interval during which it is immersed in the electric field, the intensity of the field and the thickness of the dielectric material. The optimum combinations of these factors must be determined experimentally.

"After Eguchi's experiments many investigators applied polarizing fields of the order of 1,000 volts per millimeter of electret thickness. More recently units have been formed with fields of lower intensity. Fields in excess of about 1,200 volts per millimeter do not increase the maximum charge retained by electrets. At the other extreme, electrets have been made in fields of only 10 volts per millimeter.

"As Eguchi found, immediately after the electret is removed from the field the wax surfaces carry charges of a sign opposite to the sign of the adjacent electrodes. Wax in contact with the positive electrode acquires an initial negative charge and wax in contact with the negative electrode acquires a positive charge. Andrew Gemant of the University of Oxford has applied the term 'heterocharge' to the initial effect and 'homocharge' to the field of reversed polarity that appears after the heterocharge decays.

"The terms are useful for emphasizing an important distinction between electrets formed by high voltage and those formed by low voltage. Electrets that are formed in fields of less than about 1,000 volts per millimeter of dielectric thickness usually exhibit a heterocharge that decays to some constant value within 10 to 20 days. The magnitude of the charge is proportional to the polarizing field. No homocharge appears. On the other hand, electrets prepared with polarizing fields of 1,000 volts or more per millimeter are characterized by an initial heterocharge that usually decays into the stronger homocharge.

"The unpredictable nature of electrets accounts for part of their fascination For example, it has been reported that electrets have been made by letting a mixture of wax and rosin solidify between sheets of tinfoil without a polarizing field. The charge carried by the resulting units showed no sign of decay after a storage period of six months. Stable electrets have also been formed at room temperature, far below the melting point of the dielectric material. The resulting charges were relatively weak and decayed more quickly than the ones formed in the molten state.

"The performance of the completed units cannot be increased by heating the dielectric material much above its melting point. It has been reported that the final charge is less than maximum if the polarizing field is removed before the dielectric solidifies or if it is maintained for an extended period after the dielectric solidifies. To make electrets of the highest charge I switch off the field when the dielectric has cooled to within five degrees centigrade of room temperature. "I have been interested chiefly in investigating the properties of thin electrets on the assumption that they might be more useful than thick ones in practical applications. Electrets made by Eguchi and many other workers have for the most part ranged from eight to 20 millimeters in thickness and from 50 to 500 millimeters in diameter. Eguchi's electrets were made in the form of disks. Edwin P. Adams of Princeton University developed a cylindrical type in which the dielectric was sandwiched between a pair of concentric metal cylinders.

"My electrets are rectangular, 10 millimeters square. They range in thickness from a fraction of a millimeter to five millimeters. They are formed within a rectangular perforation 10 millimeters square cut in a sheet of mica; they remain permanently attached to the mica at the edges. The mica serves as a mechanical support and a convenient fixture for handling the charged dielectric. A sheet of stiff plastic, such as Formica, could be substituted for the mica. Mica is convenient for investigating thin electrets, however, because it can be split easily into sheets of any desired thickness.

"When making wax electrets, I lay the mica over a small rectangle of tinfoil placed so that the foil is covered by the rectangular perforation, thus forming a small container with mica sides and a foil bottom. The melted wax is poured into the container and covered by pressing a second sheet of tinfoil on top. The upper and lower foils remain attached when the wax solidifies; they serve as electrodes. Electrets made of plastic such as Plexiglas, nylon and Teflon are formed in much the same way. Plastic in square sheets piled to the desired thickness is placed in the container, covered with tinfoil and pressed while being heated in an oven and during subsequent cooling.

"A simple fixture was improvised for holding and simultaneously pressing thin electrets during fabrication. The fixture consists of a multilayered sandwich of copper and ebonite fastened together at the edges by four machine screws [see Figure 1]. I made the bottom layer out of flat sheet copper 50 millimeters wide, 75 millimeters long and two millimeters thick. The lower electrode of tinfoil is placed on top of the copper at the center. The sheet of perforated mica, which is also 50 millimeters wide and 75 millimeters long, is placed on top of the tinfoil and the copper.

"The top layer of the sandwich consists of a sheet of ebonite perforated by a rectangular opening 30 millimeters wide and 40 millimeters long. This opening provides access to the cavity formed by the perforated mica and the lower tinfoil. The upper rectangle of tinfoil is pressed into contact with the mica by a copper plate 20 millimeters wide and 30 millimeters long. This plate, which is two millimeters thick, is held in place by a 40-millimeter length of flat spring steel attached at its outer end by one of the machine screws with which the sandwich is fastened.

The screw that secures the steel spring must be insulated electrically from the copper plate at the bottom. I insulated it by cutting a rectangular notch in one end of the copper plate and fitting a piece of ebonite into the opening. The ebonite member is held in place by a strip of ebonite placed under the copper plate. The binding screw passes through the middle of this strip, the ebonite insert, the mica, the ebonite top and the end of the spring. A small handle of copper rod was soldered to the copper pressure plate for convenience in handling the piece.

"The copper plates must be made as flat as possible. Mine were squeezed between the jaws of a press and then lapped with fine abrasive. Tinfoil can be flattened by sandwiching the metal between paper, placing the sandwich on a sheet of glass and stroking the paper with a pressure of a few ounces applied by a fingertip. To make electrodes without bending the flattened foil, cut the sandwich into rectangles of the required size before separating the foil from the paper.

"Make the foil that will serve as the upper electrode slightly larger than the copper pressure plate. Place this foil on a flat surface, center the pressure plate on top of it and bend the corners of the foil up over the copper. Place the foil that will serve as the lower electrode on top of the copper plate at the bottom of the fixture and adjust its position so that the foil will be covered by the edges of the perforation in the mica. If either piece of foil tends to bow into the perforation, press it back into contact with the copper plate. The production of electrets of uniform thickness requires that both foils be flat.

"To make an electret of wax, melt the wax in a clean beaker. Simultaneously heat the fixture in an oven to a temperature slightly above the melting point of the wax. An oven equipped with a thermostatic control is convenient but not essential.

"Fill the cavity of the fixture with melted wax until the upper surface bulges slightly above the upper surface of the mica. Place the upper electrode on top of the wax. The higher temperature of the pressure plate will cause the wax to melt into contact with the foil of the upper electrode. Secure the pressure plate in this position by means of the flat spring. Some wax may be squeezed out around the edges of the pressure plate. Remove it with a flat wooden toothpick or some other spatula.

"Electrets of plastic are similarly assembled. Cut the plastic into 10-millimeter squares. Place in the perforation of the heated fixture as many squares as necessary to build up an electret of any thickness. Cover the plastic with the pressure plate and the foil, apply the spring and put the assembly in the preheated oven. (The oven should be preheated to the temperature at which the plastic yields under the applied pressure. Determine this temperature experimentally.)

"Polarizing potential is applied to the fixture by a pair of leads that are insulated by a covering of asbestos. Wire of this type is commonly used in household appliances such as toasters and is available from dealers in electrical supplies. The polarizing energy must be provided by a direct-current source. I use dry batteries.

"A power supply could be improvised by rectifying and filtering the output of a step-up transformer. The rectifier can consist of a 1B3 diode connected in series with one output lead of the transformer. The filament of the diode can be heated by a 1.5-volt dry cell connected in series with a 1/4-ohm, one-watt resistor. The anode of the tube should be connected to the output lead of the transformer. Adequate filtering can be provided by connecting a high-voltage capacitor of the 'cartwheel' type (commonly used in television sets) to the remaining grounded lead of the transformer and the filament of the tube.

"To control the output voltage connect a string of 12 one-watt resistors of one megohm each across the capacitor. Connect the grounded side of the power supply to the copper plate of the electret fixture and with an alligator clip connect the top electrode of the fixture to a resistor selected according to the voltage desired. A second capacitor of the same type connected across the leads to the electret would provide additional filtering. The maximum output voltage of the power supply will be equal to the voltage shown on the nameplate of the transformer multiplied by 1.414. The polarizing potential need not exceed 1,000 volts per millimeter of electret thickness, although it is interesting to compare the performance of electrets formed at higher and lower voltages.

"A typical wax electret is formed by applying the polarizing potential and heating the fixture at constant temperature for three hours. The heat is then turned off. My oven cools to room temperature in six hours. I then switch off the power and short-circuit the leads to the electret. The tinfoil of the upper electrode is unfolded, the pressure plate is removed and the mica is lifted from the fixture with both foils intact. The electret is quickly short-circuited by wrapping a narrow strip of foil around the unit so that it makes contact with both electrodes. The foil electrodes should be removed only at the time of measuring the charge. In the case of very thin electrets the lower tin electrode should not be removed at all, otherwise the sample will come out of the mica perforation.

"The charged electret cannot perform work. The device simply supports an electrostatic field. The electret can, however, be used for generating (by induction) electrical energy that can perform work.

"To generate electrical energy in a brass disk, I connect the disk to ground and place it in contact with one surface of the electret. The other surface is grounded. Electrostatic force either drives electrons out of the disk and into the ground or attracts electrons from the ground into the disk, depending on the polarity of the electret's field.

"I then break the ground connection to the disk, thereby preventing further displacement of the electrons, and lift the disk away from the electret. Work must be performed to separate the disk from the electret against the force of electrostatic attraction; energy thus stored in the disk can be expended to perform work. The amount of energy so generated varies with the strength of the electret and with the amount of the disk's separation from the electret.

"I improvised an apparatus that consistently lifts the disk at a predetermined rate to a predetermined height. The amount of energy generated by this apparatus varies only with the charge on the electret. I measure the charge by connecting the output of the apparatus to an electrometer.

"The apparatus consists of a cylindrical container that supports on its axis a movable rod of brass terminating in an insulated disk of brass [see Figure 4]. The rod is supported at any intermediate position by a pair of adjustable helical springs and is locked at the limit of its downward excursion by a latch. When the latch is operated, the springs lift the rod to its elevated position.

"An electret to be tested is placed on the metal plate that forms the base of the apparatus; the position of the electret should be directly below the rod and its terminal disk. The rod is pushed down so that the disk makes contact with the surface of the electret; in this position the rod is latched. The external switch is operated to ground both the disk and the electret and is kept in this position for one minute to let the electret become electrically stabilize. The output of the apparatus is then switched to the electrometer and the latch is operated, thus transferring the surface charge of the electret to the disk by induction. The magnitude of the resulting pulse of energy, as indicated by the electrometer, is observed and tabulated.

"The temperature at which electrets are formed influences the charge they acquire. The two accompanying graphs [Figures 5 and 6] depict the performance of six electrets made of 75 percent carnauba wax mixed with 25 percent beeswax Three of the units were .5 millimeter thick and three were one millimeter thick. All electrets were formed by a polarizing potential of 1,000 volts per millimeter of thickness.

"In a typical case I adjusted the oven to a temperature of 80 degrees C. The fixture containing the dielectric was placed in the oven and voltage was applied. After three hours the heat was shut off. The oven cooled to room temperature in six hours. The high voltage was then turned off. Electrets were similarly prepared at 60 and 25 degrees C.

"To store electrets I wrap a ribbon of tinfoil completely around the mica holder, cover both sides of the mica with a uniform layer of absorbent cotton and place the assembly between a pair of ebonite plates bound together by a rubber band. The units are stored in a desiccator containing anhydrous calcium chloride.

"A variety of applications may eventually be found for the electret, although at present it is little more than an interesting plaything. Gemant at Oxford has used an electret as the active element of a simple electrometer. With a fine wire he suspended an electret 30 millimeters in diameter and six millimeters thick between a pair of metal plates 60 millimeters square spaced 42 millimeters apart [see illustration above]. A small mirror attached to the electret reflected a spot of light to a scale 115 centimeters from the apparatus. A potential of one volt applied to the plates caused the spot of light to move 3.5 millimeters. The sensitivity could doubtless be improved by substituting for the supporting wire a thin fiber drawn from quartz or glass.

"Alternating currents can be generated by vibrating a metal plate close to an electret. Microphones and loudspeakers have been based on this principle. Other potential applications of electrets include motors, timing instruments, hearing aids and similar devices.

"Certain dielectric materials can be polarized by exposure to light. Devices so made are known as photoelectrets and have been used for making electrostatic recordings that are analogous to recordings of sound made on magnetic tape. Because electrets made of ceramic materials show piezoelectric and pyroelectric effects, they may find application as timing devices and as memory elements in computers."

Amateurs intending to experiment with electrets will find that some of the supplies are not readily available in most localities. Carnauba wax, for example, is rarely stocked in stores. All supplies needed for experiments with electrets can be obtained, however, from Jay T. . Nichols, P.O. Box 161, Wilmette, Ill. 60091.

Bibliography

ELECTRET. E. Gutmann in Reviews of Modern Physics, Vol. 20, No.3, pages 457-472; July, 1948.

FUNDAMENTALS IN THE BEHAVIOR OF ELECTRETS. W. E. G. Swann in Journal of the Franklin Institute, Vol. 255, No. 6, pages 513-530; June, 1953.

PLASTIC ELECTRETS. H. H. Wieder and Sol Kaufman in Journal of Applied Physics, Vol. 24, No. 2, pages 156-161; February, 1953.

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