NEAR THE CLOSE OF THE 17th century Otto von Guericke, the amateur physicist of Magdeburg, invented the world's first electrical machine-an electrostatic generator. He wrote down these instructions for building one:

"Secure one of the glass globes which are called phials, about the size of a youngster's head; fill it with sulfur, ground in a mortar and melted by the application of a flame. After it refreezes, break the phial, take out the sulfur globe and keep it in a dry place, not a moist one. Perforate it with a hole so that it can spin upon an iron axle. Thus the globe is prepared.

"To demonstrate the power developed by this globe, place it with its axis on two supports in the machine-a hand's breadth above the baseboard-and spread under it various sorts of fragments such as bits of leaves, gold dust, silver filings, snips of paper, hairs, shavings, etc. Apply a dry hand to the globe so that it is stroked or grazed two or three times or more. Now it attracts the fragments and, as it turns on its axis, carries them around with it.

"When a feather is in contact with the globe, and afterwards in the air, it puffs itself out and displays a sort of vivacity . . . and if someone places a lighted candle on the table and brings the feather to within a hand's breadth of the flame, the feather regularly darts back suddenly to the globe and, as it were, seeks sanctuary there."

After describing numerous other experiments, in some of which the globe produced light and sound, von Guericke concluded: "Now many other mysterious facts which are displayed by this globe I shall pass by without mention. Nature often presents in very commonplace things marvelous wonders which are not discerned except by those who through insight and innate curiosity consult the oracle of experimentation."

Ten generations of experimenters have consulted the oracle since von Guericke's day. So fathomless are the mysteries of his sulfur ball that its "marvelous wonders" continue to charm and sometimes baffle experimenters, whether they twiddle the controls of a Van de Graaff accelerator or merely stroke the hairs on a cat's back. But electrostatics has remained chiefly a curiosity. Most people are acquainted with it only in the form of the crackling shock you get when you touch a metal doorknob after walking across a thick carpet in winter. Those who go in for amateur radio grumble that "static is something you cuss, not study!"

Static electricity has never become economically important, probably because nature has been more generous in supplying effective conducting substances and magnetic materials than she has in providing good insulators. Our electrical technology is based on electromagnetic devices; our electrical power is generated by magnets and moving conductors. Electrostatic machines have been harnessed for only a few specialties, such as generating high voltage for laboratory experiments and producing high-energy X-rays and sterilizing drugs in medicine.

Some engineers believe that electrostatics has big technological possibilities. One of them is John G. Trump, professor of electrical engineering at the Massachusetts Institute of Technology. He points out that the forces resulting from the presence of electric charge are the most direct and powerful in nature. He and his colleagues at M.I.T. are conducting certain investigations which may produce an electrostatic power generator that one day will compete with the electromagnetic generator.

Professor Trump illustrates how the power-generating capacity of electrostatic machines may be stepped up by asking you to consider two metallic plates, 100 square inches in area, facing each other and separated by an insulator.

If a voltage amounting to an electric field of 300 volts per centimeter is applied between them, the plates will be attracted to each other with a force of one 2,000th of a pound. Increase the field to 80,000 volts per centimeter and the attraction becomes half a pound. Now immerse the plates in a high vacuum-a good insulator, though one difficult to maintain-and increase the field to three million volts per centimeter. The force of attraction jumps to 5,700 pounds! "Force of this order," says Trump, "has more than passing interest for power engineers."

Trump and his associates are working on the problem of developing a practicable system of vacuum insulation which would make possible a field intensity of millions of volts per centimeter in a large machine. If they succeed, an electrostatic power generator will become a reality. The working parts of such a machine would look like an oversized variable capacitor with intermeshing leaves. Only the rotor would move. The machine could be constructed of light metal. A generator of this type about the size of a hall bedroom and weighing only a few hundred pounds, could deliver 7,500 kilowatts of power. Its efficiency would be impressively higher than that of an electromagnetic generator.

The history of electrostatics began with the discovery of Thales of Miletus that rubbed amber attracted other objects. Through the centuries experimenters like von Guericke explored the "marvelous wonders" of static electricity with a succession of ingenious machines which amateurs may enjoy building.

One of the most important historically was Alessandro Volta's elettroforo perpetuo, now known as the electrophorus [Figure 1]. Volta wrote to Joseph Priestley of the Royal Society on June 10th, 1775: "I hereby draw your attention to a body that, after being electrified by a single brief rubbing, not only does not lose its electricity but retains obstinately the indications of its active force in spite of being touched repeatedly any number of times." The electrophorus consists of two working parts: (1) a rectangular block of insulating material such as lucite or, preferably, polyethylene, and (2) a metal disk fitted with an insulating handle. When the lucite is wiped with a woolen cloth, electrons are removed from the cloth and deposited on the lucite-in much the same way that a dirty rag smears a clean sheet of glass. Early experimenters (who used sealing wax, hard rubber or other resinous substances instead of plastic) said that the insulator thus stroked had been "electrified by friction." They failed to observe that the cloth took on an equal and opposite charge. When electrons rub off from the wool to the lucite (because, as it happens, some of the atoms in wool hold electrons less tightly than those in lucite do), the atoms that have lost electrons become positively charged ions. Accordingly the surface of the wool is peppered with tiny areas of positive charge while the lucite has similar areas of negative charge. The charges are static-bound at the places where they are deposited-because in an insulator electrons cannot move freely about in the substance.

Ever since Benjamin Franklin named the "positive" and "negative" ends of the electric field, the field has been thought of as originating at the positive end. He might have prevented confusion if he had assigned the names the other . way around, because we now have to say that the current "flows" from positive to negative, although actually the electrons flow from negative to positive!

Be that as it may, the field between a pair of opposite charges is often pictured as a pattern of curving lines that radiate into space from a region deficient in electrons and converge on one where they are in excess. The field can be thought of as a bundle of stretched rubber strands, illustrating that there is a force of attraction between the charges of opposite sign.

Von Guericke demonstrated that an electrified insulator would communicate its charge to one that is electrically neutral. We now know that it accomplishes this by sharing its excess electrons with the uncharged body at points where the two touch. Von Guericke also showed that a body can become temporarily electrified merely by entering a field of charge, without touching the charged surface. He wrote: "If a linen thread supported from above is brought near the globe and you try to touch it with your finger or any other body, the thread moves away and it is difficult to bring the finger near the thread."

This was an important discovery. It demonstrated that charging by induction does not exhaust the charge on the body initially electrified, and also that charges of like sign repel each other. Charging by contact involves the sharing of electrons between two bodies, and each contact diminishes the number remaining on the charging objects. Charging by induction, in contrast, makes no demand on the free electrons but only on the field set up by them. The field causes the electrons of the uncharged substance to veer slightly from their normal orbits. This displacement sets up an "induced" charge in the previously uncharged substance. When the inductively charged body is removed from the exciting field, everything returns to normal and the induced charge disappears.

Induction can give a conductor a permanent charge, in the sense that the charge will remain on the conductor until it leaks off or is otherwise dissipated. All modern electrostatic generators are designed on the principle of inductive charging. And the charging process involves the transformation of mechanical energy into electrical energy. The principle is nicely demonstrated by the electrophorus.

When the metal disk is placed over the charged lucite, the lucite's negative field opposes that of the electrons near the lower surface of the disk. These electrons move away to the upper surface of the disk. Consequently the lower surface becomes positively charged and the upper surface negatively charged. If, while the top of the metal is thus charged, you touch the top with your finger, the excess electrons will flow into your body-where, electrically, things are not so crowded. Now if you take your finger away and then lift the metal disk by the insulating handle, a net positive charge is trapped in the disk.

This method of charging removes no electrons from the lucite, nor does it draw on the lucite's energy. Yet the metal disk is now energized with positive electricity (many protons have been denuded of their neutralizing electrons and hence their positive fields extend out into surrounding space). The disk will now attract other bodies just as the charged lucite does. Moreover, a spark will jump between the charged disk and your finger-which you can easily observe in a darkened room. The energy expended in the spark came from your muscles when you lifted the metal from the lucite. The spark was created by electrons colliding with molecules of air in their headlong rush from your body back into the disk.

A more effective arrangement for generating electrostatic energy by induction uses two Leyden jars [Figure 2]. One jar, A, has a tiny positive charge. When its positively charged terminal is brought close to a brass ball, A', electrons in the ball are attracted to the side of the ball nearest the terminal. Similarly the terminal on another jar, B, with a small negative charge, drives away electrons in the ball B', making the near surface positive. If the two balls are now connected by a metallic rod [middle drawing], electrons in B' (repelled by the field of B and attracted by that of A) will flow to A'. Removal of the rod traps the charges-just as the removal of your finger trapped those in the disk of the electrophorus. Now suppose we change the positions of the balls, moving A' toward B and B' toward A [bottom drawing]. To do this we must expend work, because A', for instance, is repelled by B and attracted by A. This work is transformed and stored as potential electrical energy as soon as we touch A' to B and B' to A. The excess electrons in A' flow into B, raising its negative charge to that of A', and A similarly acquires an increase of positive charge. The cycle can be repeated indefinitely. In theory the amount of energy stored in the jars (capacitors) can be increased without limit. In practice, the storage is limited by the fact that electrons leak away more and more rapidly as the charge increases.

Various induction machines have been designed for performing the sequence of operations automatically and with considerable speed. In these machines the "carriers" take the form of thin metallic sheets instead of balls, and the capacitors also are metal sheets, called field plates.

An early form of the machine, patented in 1860 by C. F. Varley of England, is easy to construct [see Figure 3]. It consists of a pair of field plates cemented to a square slab of lucite surmounted by a rotating disk of lucite to which six or more sectors of aluminum foil are cemented. Two brushes (of tinsel) momentarily connect opposite sectors, the carriers, with their respective field plates as each carrier enters the region of its plate. A similar pair of brushes again make contact with opposing pairs of carriers as they move from the region of the field plates. A pair of "corona" combs-quarter-inch metal rods fitted with steel phonograph needles spaced half an inch apart-graze the carriers at positions intermediate between the two sets of brushes. The machine's electrical output flows from the combs to a pair of spheres an inch or so in diameter which comprise a spark gap.

The lower left diagram illustrates the action. Assume a charge on the field plates [outer solid segments]. Electrons flow into the carrier at the left, leaving the right-hand carrier with a positive charge. Work is now expended in moving the carriers "up the potential hill" to the opposite field plates. Here they make contact with the brushes and part of their newly acquired energy flows into the field plates; electrons enter the field plate [top of drawing] from the negatively charged carrier, while the opposite carrier withdraws electrons from the lower plate. The succeeding action of all carriers is similar. After a short period of operation the combs reach ionizing potential, and energy flows from the carriers to the gap, where vigorous sparking occurs.

The machine is not very efficient. This can be demonstrated by observing its operation in a dark room. The rotating carriers appear as a blurred disk of phosphorescence in colors ranging from greenish-blue through violet, while the field plates are outlined sharply in purple. Corona discharge at the combs is $2 brilliant. This display means that electrons are streaming from the thin, sharp edges of the foil and the points of the comb carrying negative charge and into those parts carrying a positive charge. Considered as an electrical "pump," the machine is leaky and thus wastes energy.

The corona effect is explained by the geometry of the machine's conducting parts. Unless distorted by another charge, the electric field radiates into, space uniformly in all directions from a point charge. If the charge is enclosed by a conductor, the lines of force always emerge perpendicular to its surface. In the case of a spherical conductor (in effect, an enlarged point) the lines are, therefore, distributed uniformly over the surface. When the sphere is distorted to an egg shape, however, the lines bunch up at the little end and thin out at the big end-because they must emerge everywhere at a right angle to the surface. Crowding at the little end becomes more pronounced as the radius of the "point" is made smaller. This is another way of saying that the intensity of the field, or the potential gradient, increases inversely with respect to the radius of the conductor; in theory it would approach infinity at the point of a perfect needle. Even in practice, finely made points can concentrate fields of astonishing intensity. The exquisite needles used in field-emission microscopes [see "A New Microscope," by Erwin W. Müller, SCIENTIFIC AMERICAN, May, 1952] create field intensities of 750 million volts per inch in the immediate vicinity of the point-although the instrument operates from a power supply of only 5,000 volts! At this field intensity electrons are literally ripped from the metal point and ejected radially into space. Gas, if present in the tube, becomes heavily ionized. The collecting combs of the Varley machine similarly ionize adjacent air, negative charges being carried by dislodged electrons and positive charges by the ions.

In the early years of this century the Wimshurst generator, similar in basic principle to the Varley but carrying one or more pairs of disks that rotate in opposite directions, was a favored source of power for X-ray machines and other devices requiring relatively small amounts of current at high voltage. The largest machines carried as many as 12 pairs of disks seven feet in diameter and delivered potentials on the order of 200,000 volts.

The Wimshurst and other electrostatic generators of this era could not reach the million-volt range. By 1920 they had been largely replaced by electromagnetic induction coils and transformers as sources of high-voltage power.

The modern era of electrostatics began in 1929. In that year Robert J. Van de Graaff, a young Rhodes scholar from Oxford University who was working at Princeton as a National Research Fellow, invented the electrostatic belt generator which is now known around the world by his name. He was interested in developing a steady constant-potential voltage with which to accelerate atomic particles to bombard nuclei in order to obtain evidence of their internal structure. Today the Van de Graaff accelerator can be found in nearly every large nuclear laboratory in the world; it is the work horse for precision research in this field. The accelerator has attained a particle energy of more than eight million volts, and this figure may soon be more than doubled. In smaller sizes the machine has found a wide variety of applications, particularly as the power source for the high-voltage X-ray treatment of disease.

One of the nicest features of the Van de Graaff generator is its relative simplicity and low cost. Robert W. Cloud of the High Voltage Research Laboratory at M.I.T. has designed a small version as a special construction project for amateurs [see drawing on next page]. Its action is as simple as its design. A motor developing 3,000 revolutions per minute is housed in a coffee can. It drives a gum-rubber belt which passes over an insulated pulley inside the upper terminal. Spray screens, counterparts of the Varley machine's collecting combs, are situated close to the surface of the belt at each end of its run, and each connects with the respective terminal. As the machine goes into operation, frictional contact removes electrons from the belt at the driving end and deposits them on a plastic pulley. Positive charges resulting at the sites on the belt which have thus lost electrons are then carried by the belt to a metal pulley at the other end above. Electrons flow from the metal pulley onto the electron-deficient belt. As the machine continues to run, heavy charges build up on both pulleys. After a few seconds or minutes, depending on the humidity of the air, the field originating at the pulleys reaches ionizing intensity in the vicinity of the spray screens. Electrons are then withdrawn from the upper terminal and sprayed on the belt at the beginning of its downward run. Similarly electrons en route down the belt come within the region of ionization at the lower spray screen and flow by way of its supporting bracket into the lower terminal. Through this pumping action the belt continuously exhausts electrons from the upper terminal and discharges them into the earth through the lower one. This leaves the upper terminal with a net positive charge which, because of mutual repulsion of the positive "holes," distributes itself uniformly over the terminal's outer surface. Accordingly the inner surface carries no charge. In theory, voltage across the upper and lower terminals increases without limit. As in the case of the Varley and Wimshurst machines, however, the charge is limited by the quality of the insulation. At about 100,000 volts charge leaks as corona from the upper terminal and as conduction current down the insulating column at a rate equal to the two microamperes which the belt is able to carry into the terminal. Although 100,000 volts is an impressive value, the machine creates no shock hazard because the capacity of the upper terminal to store charge is small.

If a well-rounded object is brought within an inch or so of the high-voltage terminal, a spark will jump. In this type of discharge the air is rapidly changed from a good insulator to a conductor and the spark completely discharges the terminal. Reduction of the terminal voltage permits the air to regain its insulating strength, and the terminal is recharged by the belt. If an object with sharp edges is brought near the terminal, it steadily drains charge by corona and decreases the potential. Such a device, with adjustable spacing, is often used in Van de Graaff generators to maintain a constant terminal potential.

Several manufacturers, including the South Research Laboratories, the Combosco Scientific Company and the American Electrostatic Company, now market small air-insulated machines, similar to the one described, for demonstration purposes at prices from $25 to $100.

A continuous source of high direct current voltage invites endless experiments. One of the most amusing is the "jumping ball" demonstration. A half dozen small balls made of pith or other light material are given a conducting surface of soot or graphite. They are placed in a cage, which may be made of a strip of transparent plastic rolled into a cylinder and capped with tops from peanut-butter jars. The caps are connected with the terminals of the Van de Graaff. As the machine goes into operation, the electrostatic field from-the upper cap attracts the balls. They hop up to it, deliver their load of electrons, fall back and repeat the cycle as long as power is supplied.

Support a sewing needle by an insulator and connect it to the machine's upper terminal. Molecules of ionized air will rush from the point as though they were streaming from a jet under pressure. They easily blow out a match or candle. This electric wind can be made to drive a simple motor. Cut a swastika, with sharply pointed tips, from aluminum foil and indent its center with the pointed end of a pencil. Pivot the indentation on the point of a pin which has been thrust up through a supporting base of cardboard. The swastika will then be free to rotate on the pin point. It will do so vigorously if the pin is connected with the high-voltage terminal of the Van de Graaff. Ionized air streaming from the four points sets up the reactive force of a jet engine.

The power capacity of the Van de Graaff is enough to charge a person to about 50,000 volts. This is 20,000 volts above the ionizing point of air at atmospheric pressure. It is also enough to make the experimenter's hair stand on end. To demonstrate this effect, stand in or on a large glass bowl or a wooden platform supported by four square milk bottles. Touch the high-voltage terminal. After a few seconds your hair will slowly rise. Incidentally, the body adds capacity to the terminal, and so a somewhat larger charge than normal accumulates. When you step down or touch a grounded object, you will experience a slightly painful shock-but it is not dangerous to a person in normal health.

Fluorescent lamps will light up brilliantly where they are touched to the high-voltage terminal. If the room is not too brightly lighted, filament-type lamps also will glow in various colors depending on the kind of gas they contain. You can even manufacture a miniature aurora borealis by boiling water in a thin flask until the air is displaced by steam and then stoppering it immediately. After the steam has condensed, the rarefied air inside will glow greenish and pink when the flask is brought in contact with the Van de Graaff.

Those who can blow glass and exhaust it-or who can induce a local manufacturer of advertising signs of the glowtube type to do so for them-may want to try their hand at assembling and operating a linear accelerator and related apparatus used for nuclear research. Such projects are on a par with amateur-built cyclotrons and, like marriage, are not to be entered upon lightly. They are, nevertheless, well within reach of amateur resources, particularly for groups.

To power such an apparatus you will require a larger version of the Van de Graaff [Figure 5 ]. It differs from the low-power design in a number of subtle, though important, particulars. The spray points for charging the upward run of the belt are supplied by a potential of 5,000 to 10,000 volts from a transformer-rectifier combination. The high-power machines employ metal pulleys at both ends of the belt, the upper one being insulated from the high-voltage terminal.

Charge is sprayed onto the belt as it passes through the corona between the lower points and the grounded driving pulley. A similar set of points, located just inside the upper terminal, removes charge from the upward belt run and conducts it to the upper pulley. After a short period of operation the upper pulley acquires a high charge and current flows to the upper terminal through a current-regulating resistor. This circuit may also include a corona gap near the inner surface of the terminal. A second set of spray points (charging rod), connected directly to the high-voltage terminal, is situated at the top of the pulley. The difference in potential between the upper pulley (made "live" by the voltage drop across the current regulating resistor and corona gap) and the high-voltage terminal causes these points to spray a charge of opposite sign onto the downward run of the belt. The value of the current-regulating resistor is chosen so that both sides of the belt work equally. The value of the current-regulating resistor can be computed roughly by Ohm's law. Belts for high-power machines are usually made of rubberized fabric and run at speeds of 4,000 to 6,000 feet per minute.

The capacity of the upper terminal to store charge varies with its size. Its ability to hold charge varies with shape. The ideal terminal would be spherical. Unfortunately this ideal cannot be realized because provision must be made for the entry of the belt. The shape must be such that the intensity of the field at the high-voltage terminal is always less than the value at which spark or corona discharge occurs. Hence the aperture of the terminal must have re-entrant edges and the facing sides of the upper and lower terminals should be identical. Such terminals are commonly made of aluminum spinnings.

Large Van de Graaffs in the million-volt range, intended for scientific and industrial purposes, are now nearly all mounted within a steel tank containing Freon, carbon dioxide or a similar gas at many atmospheres of pressure. The high pressure serves to increase the voltage-insulating ability of the gas many-fold and thus increases both the voltage and current capacity of the machine. Such machines are marketed by the High Voltage Engineering Corporation

When Van de Graaff machines are designed for potentials above 200,000 volts, the distribution of charge along the insulating column (and even along the belt runs) becomes important. The columns of air-insulated machines using belts more than four inches wide should be fitted with equipotential rings spaced along the insulating column at intervals of about two inches.

Anyone undertaking the construction of a high-power Van de Graaff should remember that he is building no toy. These potentially lethal machines can reveal "marvelous wonders" beyond von Guericke's most inspired imagining, but, unlike his sulfur ball, they pack the wallop of lightning!

Bibliography

ELECTROSTATIC SOURCES OF ELECTRIC POWER. John G. Trump in Electrical Engineering, Vol. 66, No. 6, pages 525-534; June, 1947.

ELECTROSTATIC SOURCES OF IONIZING ENERGY. J. G. Trump in Transactions of the American Institute of Electrical Engineers, Vol. 70, Part 1, pages 1021-1027; 1951.

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