Experiments Which Show That the Earth Functions As an Electrostatic Machine
by C. L. Stong-->-->
ONE SUNNY MORNING last fall a jolt of electricity knocked Fred Ellis, an Ohio radio ham, from the roof of his barn. Fortunately Ellis landed on a stack of straw and lived to tell the tale. "I started to hook a ground wire to a 50-foot antenna tower on the roof," he writes, "when a fat spark jumped from the metal tower to my hand and knocked me off balance. Although the weather was hot and dry there had been no breeze of any consequence for several days. Hence the charge could not have been generated by wind friction. How do you account for it?" In Minneapolis a steeplejack was badly burned earlier in the year when a similar spark jumped from the base of a flagpole and touched off a nearby pile of gasoline-soaked rags. Again the accident occurred on a clear, still day. A television viewer on a Pennsylvania farm, plagued for months by a strange electrical disturbance which occasionally obscured his picture, finally cured the difficulty when he detected small sparks jumping from the sheet metal cupola on his house. The trouble was ended by grounding the cupola. In all three cases the explanation has to do with the sometimes-overlooked fact that the earth is, among other things, the negative electrode of a huge electrical generator of the Van de Graaff type. Many details of this natural generator await explanation; if you enjoy electrostatic experiments, here is a readymade project and a machine that goes with it.
Figure 1: Principal components of nature's Van de Graaff generator
Nature's generator works on the same basic principle as the man-made Van de Graaff machine-but differs remarkably in its geometry and details of construction. The conventional Van de Graaff employs a pair of spherical electrodes, held apart by a cylindrical insulating column and charged by the action of a motor-driven belt. The arrangement resembles a big dumbbell. The belt "pumps" electrons out of one sphere and deposits them in the other. As the pumping action continues, an electric field builds up between the spheres. The voltage is limited only by the curvature of the collectors and the insulating properties of the column, belt and surrounding atmosphere. In nature's design, on the other hand, the parts of the generator take the form of concentric spheres. The earth, which is the negative electrode, is at the center. It is surrounded . by a hollow sphere of insulating material: the lower atmosphere. The insulator is in turn surrounded by the positive electrode: the ionosphere. A spherical pattern of thunderstorms within the insulator functions as the charging belt. The machine's effectiveness as a generator of high voltage leaves something to be desired because the atmosphere is a relatively poor insulator: its resistance amounts to only 200 ohms. Yet thunder- 6,;. storms do an astonishingly good job of keeping the electrodes charged.
Lightning, the counterpart of the . man-made machine's corona points, carries current amounting to thousands of amperes. On the average 100 strokes of lightning hit the earth every second. You can hear the resulting electromagnetic debris at any hour by hooking a long antenna to the input of an audio frequency amplifying system and turning up the volume [see "The Amateur Scientist; SCIENTIFIC AMERICAN, January, 1956]. It makes a sharp, buzzing s;. sound. The energy output of the 2,000 . to 6,000 thunderstorms in progress at any one time is sufficient to maintain an average potential of 360,000 volts between the earth and the ionosphere. This despite a leakage of current through the atmosphere of 1,800 amperes! The intensity of the field diminishes with height, but near the surface it amounts to about 100 volts per yard of altitude. Accordingly we walk around with our heads in air some 200 volts positive with respect to the ground at our feet [see "The Earth's Electricity," by James E. McDonald; SCIENTIFIC AMERICAN, April, 1953].
Figure 2: Distribution of charge in a thunderstorm
Given sufficient time, conductors insulated from the ground pick up the potential of the surrounding air. A negatively charged body attracts positive ions from the air until its negative charge is neutralized. Conversely, a positively charged body attracts negative ions. The time required for a body to reach electrical equilibrium with the surrounding air depends largely on its size and the concentration of ions naturally present in air. The amount of electricity that a body can accumulateits capacity for holding chargeis also proportional to its size. Ellis's antenna tower was so big, and had lost so much negative charge to the surrounding air, that he was knocked off balance by the swarm of returning electrons. Had he touched a small object, say a metal ventilator or several hundred feet of wire, the current would doubtless have been so small he would not have felt it.
Several methods of measuring the intensity of the earth's field are based on the fact that conductors can reach electrical equilibrium with the surrounding air. These methods lend themselves to an interesting series of experiments. In principle the measurement is simple. You merely place a test structure ( a few feet of wire will do) at a given height above the ground, insulate it from the earth, wait until it reaches electrical equilibrium with the air, and connect a voltmeter of appropriate sensitivity between the test structure and the earth. The meter reading is divided by the height in feet; the result is the field intensity in volts per foot. The field intensity varies considerably with location, topography, the hour of day and the state of the weather. Readings made at the top of mountains or steep hills average much higher than those made at sea or in prairie country. Conversely, observations made in valleys average somewhat lower. Usually the charge of the earth is negative, but under the trailing edge of a thunderstorm it is often positive. Here potential differences of 3,000 volts per foot have been recorded. The intensity of the earth's electrical field also varies during the day; this appears to be a consequence of the uneven distribution of the earth's land area. The number of thunderstorms reaches a peak when the continents of the Western Hemisphere are turned toward the sun; at that time Europe and Africa have just absorbed their daily quota of solar heat. This explanation, like many others concerning the generation and distribution of the earth's electric field, has not been tacked down as neatly as physical meteorologists would wish. Mori observations of the daily population of thunderstorms, together with corresponding information on the variation in field intensity, are needed.
Figure 3: Diagram of the earth's electric circuit
Although the measurement of field intensity is simple in principle, it is made difficult in practice by the fact that considerable time may be required for a conductor to pick up charge from the surrounding air. Lord Kelvin devised ,' the first method of speeding up the process. He insulated a metal tank from the ground, filled it with water and arranged matters so that water would flow from a spray nozzle at the bottom, where the charge density is greatest per unit area. He called the arrangement a "water-dropper." The surface of each drop, immediately before it is detached from the stream, is an extension of the surface of the tank. Accordingly the potential difference between the drop and the air is equal to that between the tank and the air. When the drop is detached, it carries charge away. The process continues until equilibrium is established between the tank and the air. The test conductor need not consist solely of the tank. A relatively small water-dropper can be used for charging a test structure of any desired size. Garden sprays of the type which operate on compressed air supplied by a hand pump will charge a 30-yard length of wire in about 20 seconds. Fine spray is preferred to large drops because the higher ratio of droplet surface to volume not only conserves water but speeds the charging process.
The charging time can also be reduced by enriching the concentration of ions in the vicinity of the test structure. For example, a wire 10 to 15 feet long with a glass insulator at each end is suspended a yard or so above the ground between a pair of posts. A short length of the wire is then wrapped in string or paper that has been soaked in inflammable lead nitrate. When the string or paper is ignited, the flame produces ions in the vicinity of the wire; in a few seconds the potential of the wire will reach that of the ambient air. A torch waved in the vicinity of the wire will accomplish the same result but at some cost in terms of convenience. Where it is desirable to monitor the field continuously over long periods, radioactive materials may be called into service. A film of polonium electrolytically deposited on a wire will emit alpha particles which strongly ionize the surrounding air.
The practical measurement of field intensity is further complicated by the necessity of using measuring instruments of extreme sensitivity. Test conductors of reasonable size store little energy at low voltagesat most a small fraction of a watt. If a substantial part of that little is consumed in driving the pointer of the measuring instrument, the accuracy of the reading will suffer. Lord Kelvin measured the potential of his water-dropper with a quadrant electrometer. This instrument consists of a metal vane suspended by a fine wire inside a metal structure which resembles a pillbox cut into four sectors. All the sectors are insulated from the ground, and sectors diametrically opposed to each other are wired together. The two pairs of sectors are then charged in opposite polarity by a battery, and the vane is connected to the test structure under measurement.
A small mirror cemented to the wire from which the vane is suspended serves as a pointer; a spot of light reflected from the mirror moves as the mirror turns. The vane moves in response to the electrostatic attraction and repulsion between its charge and that of the sectors. In the finest quadrant electrometers the wire supporting the vane is replaced by a quartz fiber which has been made a conductor by depositing a film of gold on it. When the vane is charged, it draws almost no current; yet the device is extremely sensitive to changes in voltage. Amateurs who go in for instrument making will find the construction of such an electrometer a challenging and rewarding enterprise.
Most modern electrometers are built around vacuum tubes of the screen-grid type. The basic circuit is simple. Bias voltage is connected between the cathode and grid of the tube through a high resistance. A microammeter is inserted into the plate lead; an ordinary battery supplies both screen-grid and plate voltage. The test structure is connected to the grid and the cathode is grounded. The intensity of the earth's field determines the charge on the grid and hence the strength of the plate current. The device is calibrated by connecting a source of known voltage to the grid and noting the resulting; deflection of the microammeter.
Figure 4: Van de Graaff generator with internal belt and external excitation
A number of precautions must be observed or the grid will draw an intolerable amount of current. Some current, for example, is contributed by positive ions which are ejected by the cathode and strike the grid. Another source of current is gas atoms ionized by electrons accelerated between the grid and plate. In addition, some of these electrons strike the plate with enough energy to produce soft X-rays, which in turn dislodge electrons from the grid. Light falling on the grid also ejects electrons from it. Finally, grid current tends to leak along the surface of the bulb.
A number of techniques have been developed for reducing these currents. The production of positive ions can be reduced by operating the cathode at subnormal temperature. The grid can in effect be shielded by charging the screen grid to a higher positive potential (15 volts) than the plate (8 volts). This has the effect of repelling positive ions ejected by the cathode. Reduced plate voltage prevents electrons from acquiring enough energy for ionizing the residual gas in the tube and lowers the production of X-rays. Photoelectrons are eliminated by enclosing the tube assembly in a light-tight box. Surface leakage is reduced by washing the glass envelope with alcohol and dipping the area surrounding the terminal of the control grid in ceresin wax. In applications demanding maximum sensitivity air is evacuated from the light-tight box.
By meeting these circuit requirements and using a tube of special design, such as the-General Electric FP-54, it is possible to detect currents of 60 electrons per second. Other special electrometer tubes include the Victoreen 5803 and Raytheon 5886. The RCA 954 and 959 and the Western Electric 259-B types may also be used in all but the most exacting electrometer applications. For maximum sensitivity the grid must be tied to the cathode through a resistance on the order of 100,000 megohms. Such resistors are manufactured by the S. S. White Company, Victoreen and the International Resistance Company.
Figure 5: Van de Graaff generator with external belt and internal excitation
A third method of measuring the earth's field intensity eliminates the need for highly sensitive instruments by employing a device called a "field mill," which in effect transforms electrostatic charge into alternating current. Alternating current from the mill is amplified electronically and rectified. The output drives low-sensitivity direct-current ammeters, recorders and so on as desired. In effect, the field mill is comprised of a motor-driven variable capacitor which is connected in series with the capacitor represented by test structure, the earth's surface and the intervening air. As the capacity of the variable capacitor increases, current flows into it from the test structure. When the current of the variable capacitor decreases, current returns to the test structure. The frequency is made high in relation to the time required for the test structure to assume electrical equilibrium with the surrounding air. Hence the test structure remains substantially at the ambient potential, and energy lost through the weak oscillating current is not significant. Output from the mill is provided by voltage developed across a megohm resistor inserted between the variable capacitor and the test structure.
As usually constructed, the mill takes the form of a capacitor with three plates, two fixed and the third motor-driven. One plate is a shallow cylinder closed at the bottom and open at the top. The other two plates form the lid of the cylinder. Each of these plates looks rather like a two-bladed paddle; it consists of opposing quadrants joined at the apex One of them is mounted flush with the open end of the cylinder, but is insulated from it. This plate covers two quadrants of the cylinder, leaving two quadrants open. The open quadrants are-closed by the other plate, which is mounted directly above them. This last plate is keyed to the end of a shaft which extends through the assembly to the motor, fastened to the bottom of the box. When the shaft rotates, the blades of the plate alternately cover and expose the fixed plates. The rotating assembly is mounted on insulators and grounded through the megohm resistor across which the output voltage is developed. The alternating-current output is rectified by a synchronous reversing switch, or commutator, keyed to the opposite end of the motor shaft. By rectifying the output synchronously, both field polarity and intensity can be observed. The mill is calibrated by comparison with a known voltage. It is both rugged and portable. Special designs have been developed for use in aircraft, and have contributed significantly in recent years to this phase of meteorological research.
Franklin B. Lee, a chemical engineer and faculty member of the lrie County Technical Institute in Buffalo, N. Y., has built a number of small Van de Graaff machines and passes along the results of his experiments to amateurs who prefer to work with generators less bulky than the earth. "Two important pieces of information for the designer of Van de Graaff machines," he writes, "are, first, approximately 50 square inches of belt per second passing over the pulleys will produce one microampere of current; second, the maximum potential developed by the machine will be equal to 70,000 times the smallest radius of curvature of the collector in inches. Thus a perfect sphere 12 inches in diameter will have a theoretical limiting potential of 420,000 volts. Holes made in the sphere to admit the belt assembly alter the pattern of the field and reduce the theoretical maximum. The most effective compromise with the ideal shape for a practical collector is a spheroid slightly flattened at the bottom, with minimum radius of curvature located at a reasonable distance from the insulator to discourage sparking along the insulator surface [see "The Amateur Scientist"; SCIENTIFIC AMERICAN, April, 1955].
"The designer's choice of maximum voltage determines the size of the collector. To realize a large fraction of the theoretical limiting potential, the collector must be at least two or three diameters removed from other metallic parts. The distance should be greater if sharp-edged metal parts are present, and may be somewhat less if all parts are covered by a rounded metal shield of large radius of curvature. The opening which admits the belt to the collector ought not to be much larger than half the diameter of the collector and should be smoothly curved inward, generous radius of curvature.>
"Although a high polish adds to the attractiveness of the collector, it is not essential. Minor surface imperfections, if well rounded, limit the maximum voltage only slightly. Sharp edges or burrs must be ground down. Lint and dust particles will reduce the voltage to 40 per cent of the theoretical maximum if they protrude from the surface as much as 1 per cent of the radius. Unpolished commercial aluminum spinnings, free of lint and dust, will collect about 85 per cent of the theoretical maximum. A high polish will increase the voltage another 1 per cent. The concentration of charge around the hole through which the belt enters accounts for the remaining 14 per cent, a quantity which varies, of course, with the size of the hole and its distance from other conductors.
"The hemispheres from which my collectors have been assembled were procured from a local metal-spinner who, if not too busy, will turn them out for $4.50 each, a reasonable price considering the fact that he pays $2 for the 14-gauge blanks. I had to furnish the wooden die on which the parts were spun. They would otherwise have cost on the order of $25. Incidentally, I will be glad to supply any number of 12-inch hemispheres at cost plus postage plus 75 cents per pair for handling, boxing and so on.
"Selection of the desired current output determines the size of the belt and the speed at which it must run. Meeting this specification is not so simple as it might seem offhand, because the properties of the materials used for the belt and its driving assembly enforce speed limits on both the belt and the shaft bearings. For maximum current one should in theory use the highest possible belt speed. But there are disadvantages in running belts faster than about 100 feet per second. Higher speeds aggravate the tendency of belts to fray at the edges and to come apart at the splices. At high speeds, particularly in the case of small pulleys, extreme tension must be maintained; this leads to bearing problems. Lubrication difficulties limit the shaft speed of sleeve bearings to about 5,000 revolutions per minute. The noise level of ball bearings becomes annoying above this speed unless special steps are taken to minimize it. At belt speeds above 100 feet per second appreciable amounts of power are lost through air friction. Finally, part of the charge appears to be 'blown, off the belt at excessive speedsa phenomenon which I do not wish to be called on to explain.
"Belts may be made of almost any insulating material: paper, cloth, rubber, plastic and so on. Rubber, because of its poor resistance to ozone, has a h limited life, but with used inner tubes 't costing so little the inducement to improve on it is slight. Rayon, nylon, Dacron and cloth (made into belts with acetone cement ) are almost as good. Incidentally, when these materials are substituted for rubber, the position of the corona-collecting combs must be shifted. Cloth belts are more durable than rubber, are quieter and require less driving power, but tend to fray at the edges. This is easily remedied by a coat of lacquer. All things considered, I find that belts of neoprene joined with a diagonal splice are a good compromise.
"The upper pulley must be made of a material which is a good electrical c0nductor, such as wood or Bakelite Surprised? At such voltages these materials are very good conductors for the small currents involved. Scrap plywood may be glued together to make a highly satisfactory wooden pulley. The lower pulley should also be electrically conducting if. a separate 5,000 to 10,000-volt d.c. power supply is used for spraying charge onto the belt. If the machine is to be self-excited (that is, if the belt is to be energized by friction), the lower pulley should either be coated with or constructed of a material of extremely high resistivity. A 1/32-inch thickness of polyethylene makes a splendid covering for small wooden pulleys. The pulleys should be turned with a slight crown, the edge making a snug fit with the inside of the polyethylene tube. The tube may be made by cutting the ends off a round squeeze bottle. The tube is simply pushed over the wooden core. The choice of pulley and belt material for self-excited machines determines the polarity of the collector charge. A rubber belt running on a lower pulley of polyethylene or polystyrene will usually pump electrons from the collector and hence charge it positively.
"The belts may run on either the inside or the outside of the insulating support. Economy, simplicity and high currents favor running them on the outside. Appearance and neatness of construction require them to be on the inside. The former arrangement permits the use of a small, relatively inexpensive insulator with low current leakage and minimum deterioration due to corona discharge (the source of ozone). It also permits use of the widest belt possible for a given opening in the collector. Problems arising from unequal potentials throughout the insulator are similarly minimized. All hygroscopic or fibrous materials should be avoided in the selection of the insulating column because they invite leakage through the moisture which forms on the surface. The material must also be selected with an eye to its mechanical properties: strength, stiffness and toughness. FinalIy, it should be readily available at a reasonable price. Tubing of polyester glass-fiber laminate or polyvinyl chloride meets these requirements and is available in standard pipe sizes. In the three-inch (diameter) size, it costs $2.60 per foot. Polyvinyl chloride is the less hygroscopic of the two and may be flanged or formed to other contours by heating it to its softening point in hot paraffin.
"Combs or corona points for applying charge to the belt may be contrived in great variety. Often a common pin or a single phonograph needle can be as effective as the most elaborate comb. A tuft of wire, bound at one end and sheared like a broom at the other, makes h a satisfactory comb, as does a small rectangle of wire screening. Care must be taken to avoid spraying areas of the belt with unwanted charge. This may happen if charges are permitted to mix on the front and back of the belt near the pulleys. The problem is met by mounting the combs on fixtures which provide easy adjustment over a wide range of positions, and by selecting comb sizes which restrict the areas that are sprayed with charge."
Figure 6: a simple equatorial mounting for a small telescope
The simple Newtonian telescope that is shown at the right is submitted by W. Gorrell, Jr., of Buffalo, Wyo. A local school, having purchased a telescope kit and made a three-inch mirror, was stumped for an adequate mounting. On appeal, Gorrell solved the problem by a visit to a local automobile junkyard plus a few evenings of labor. The gadget he picked up in the junkyard should interest any amateur on the verge of assembling a small equatorial mounting.
The lucky find is a "ring-gear carriage," a component of the transmission h assembly in most pre-automatic-shift cars. "The holes through the yoke end," Gorrell writes, "are aligned precisely and make a snug fit with a .75-inch shaft. The bottom of the part is faced off. Both the outside and inside are turned. The shoulder and the top surface are nicely faced. I used only one of the inside bores (the smaller one), for which I turned a piece of pipe to fit. A brass collar on the pipe takes the downward thrust of the instrument.
"The ring-gear carriage turns in a short length of pipe, which is welded to an angle-iron base as shown. The 45-degree angle of the base is close enough to that of our latitude (44 degrees, 21 minutes) so that a slight tilt of the tripod compensates for the difference.
"Since the entire rig weighs only a few pounds and the center of the yoke is the center of balance between the 'scope and the counterweight, the instrument can be lifted off the tripodyoke, counterweight and alland reassembled without touching a single screw or clamp."
AMATEUR TELESCOPE MAKING: BOOK I. Edited by Albert G. Ingalls. Scientific American, Inc., 1951.
ELECTRONICS: EXPERIMENTAL TECHNIQUES. William C. Elmore and Matthew Sands. McGraw-Hill Book Company, 1949.
PHYSICAL METEOROLOGY John Clark Johnson. The Technology Press and John Wiley & Sons, Inc., 1954.