At the turn of the century many physicists held to the notion that electricity behaves like a fluid, that the amount of charge on an electrode can be altered by any desired amount. But some argued otherwise. For example, the emission of electrically charged particles by zinc exposed to light (the "photoelectric effect"), and the production of ions in a gas-discharge tube, supported the notion that electricity comes in tiny particles. The British physicists C. T. R. Wilson and H. A. Wilson had independently attacked the question by observing the response of a cloud of water droplets to X-rays and an electric field. Millikan decided to sharpen the experiment by concentrating on the behavior of individual drops instead of the whole cloud.

To form the drops he set up a modified version of C. T. R. Wilson's cloud amber: a glass cylinder equipped with piston. When the piston was pulled down quickly, expansion lowered the temperature of humid air inside the cylinder and caused a cloud of water droplets to form. Millikan fixed electrodes inside the chamber at the top and bottom and connected them to a 4,000volt storage battery. He lighted the droplets from the side with an arc lamp and observed them through a small telescope. When the chamber was expanded and the field applied, most of the droplets drifted to the bottom under the influence of gravity. Some of the droplets, however, picked up electric charge during the expansion; most of these promptly darted toward one electrode or the other, depending upon the sign of their charge. A scattered few of the droplets hung in space for a second or two, their tendency to fall being precisely balanced by the upward tug of the electric field. Then evaporation would reduce their weight enough to upset the balance and start them moving toward the upper electrode. It was these droplets that caught Millikan's interest. He soon learned how to compensate for the evaporation by gradually lowering the voltage across the electrodes.

Occasionally he found it possible to make a drop stand still for as long as a minute. Now and again he observed that one of these stationary drops would take off abruptly toward one or the other of the electrodes. Moreover, a drop that had started to move in this marmer would occasionally pick up speed during its flight, or stop in its tracks. Millikan therefore suspected that the drops must be picking up or losing elementary charges, perhaps the "atoms of electricity" for which the British physicist G. J. Stoney had suggested the name "electron" in 1891. "This experiment," Millikan later wrote in his book The Electron, "opened up the possibility of measuring with certainty, not merely the charges on individual droplets as I had been doing, but the charge carried by a single atmospheric ion. By taking two speed measurements on the same drop, one before and one after it had caught an ion, I could obviously eliminate entirely the properties of the drop and of the medium and deal with a quantity that was proportional merely to the charge on the captured ion itself."

In the fall of 1909 Millikan undertook a new series of experiments with a modified apparatus. The cloud chamber was replaced by a simple container fitted with windows and a pair of flat metal plates spaced about 5/8 inch apart. Light oil was sprayed into the top of the chamber by an ordinary atomizer. Some of the droplets found their way into the space between the plates through a pinhole in the middle of the upper plate. These droplets were lighted and observed by the same technique used with the water drops. The oil drops reacted like the water drops to an electric field established between the plates, but did not evaporate perceptibly during the period of observation. By manipulating the field to compensate for charges picked up from the air, it was possible to keep a selected drop under observation indefinitely. Within a matter of months the new apparatus enabled Millikan not only to prove the existence of the electron but also to measure its charge with fair accuracy and, incidentally, to measure the number of atoms in a gram of hydrogen. Before the series of experiments had ended he had independently established the approximate mass and size of the electron, confirmed Albert Einstein's explanation of the photoelectric effect and derived the value of Planck's constant experimentally! Few experiments have been more productive -or more fascinating to those who have repeated it.

One of these is George O. Smith of Highlands, N. J. Smith writes: "It may come as a pleasant surprise to a generation accustomed to thinking of atomic research in terms of cyclotrons and other complex apparatus that Millikan's equipment is easy to reproduce yet precise enough to challenge the most advanced instrument-maker.

"The experiment is based on the principle of balancing against the force of gravity the electric force acting on a small drop of oil. The electric force is determined by the interaction of the charge on the drop with the charge on the electrodes of the apparatus. The w charge on the electrodes can be measured directly with a voltmeter. But the charge on the drop must be determined indirectly by the speed with which the drop moves through the air. The speed varies with the intensity of the charge and the size of the drop, so the size of the drop must be found. This can be done by observing the maximum speed, or 'terminal velocity,' of a drop in free fall through the air. The terminal velocity of a drop is reached when the force set up by friction between the drop and the air equals the force of gravity. The two forces act in opposite directions and cancel, with the result that the speed of the drop remains constant. Since the force of gravity is known, and the terminal velocity can be determined by timing the drop in free fall through a known distance, the size of the drop can be calculated by means of a simple formula that takes certain properties of the air into account. The charge on the drop can then be calculated by inserting the known values in another simple formula and doing the arithmetic.

"The accuracy of the method depends on the care taken in the construction of the apparatus. The electrodes are enclosed in a housing that can be rectangular [see illustration below], or cylindrical. The sides of the housing can be made of glass, lucite or any similar transparent material. Lucite is easier to work than glass, but has no functional advantage. The dimensions of the enclosure are not critical. One measuring some seven centimeters wide, 10 centimeters long and 15 centimeters high is adequate. The space between the electrodes should not be more than about an eighth of the height. The electrodes should make a loose fit with the inner walls of the housing so that they can be removed easily for cleaning. The fit should not be so loose, however, that air can circulate freely between the compartments of the housing.

"The upper electrode is supported by a pair of lucite blocks. These may be cut from a slab of lucite about a centimeter thick. In addition to acting as spacers between the electrodes, the blocks serve as windows through which the drops can be lighted from the side. The ends of the blocks should make an easy fit with the walls of the box. Their width is not important. Their outer and inner sides must be polished sufficiently to permit good light transmission. The rough faces can be first smoothed against successively finer grades of garnet paper supported on a flat surface, then polished against crocus cloth until all scratches disappear, and finally finished against wrapping paper charged with rouge of the sort used to polish glass. The optical quality of the polished surfaces need not be better than that of window glass.

"Plates for the electrodes can be cut from any kind of metal. Brass is preferred. The facing surfaces of these plates must be flat to better than a hundredth of a millimeter. The upper plate must be thick enough to prevent sagging. The facing surfaces can be smoothed against crocus cloth supported on a flat surface, and finished against wrapping paper charged with rouge. The corners and edges may be rounded slightly by grinding them against the finest grade of garnet paper. A pinhole approximately a millimeter in diameter is drilled in the center of the top plate and the burs removed. The use of heavy stock will be appreciated when the apparatus goes into operation, because the weight of the upper plate holds the electrode assembly together. (The experimenter will spend a good part of his time cleaning oil from the apparatus. He is therefore advised to resist the temptation of drilling, tapping, screwing and otherwise fastening the apparatus together in a solid but time-wasting structure.) The heavier the plate is made, the less it is likely to be displaced during an experimental run. Electrical contact with both plates is made through flat springs as shown in the accompanying illustration [left]. The box is closed at the bottom by the lower electrode and at the top by a third metal plate.

"An observation port is drilled in front of the box at the level of the electrode space and fitted with a microscope cover-glass. The glass is held in position by a retaining ring of piano wire. A hole for the atomizer is drilled near the top of the box; holes are also drilled through the metal cover and the upper electrode for a thermometer. The bulb of the thermometer should be located as far as possible from the pinhole in the center of the upper electrode. The scale of the thermometer should also be kept away from the oil spray to avoid fogging.

"After the assembly is completed, the space between the electrodes must be measured. Any distance on the order of a centimeter that chances to come out in the construction is satisfactory. But this distance, whatever it may be, must be determined to the extreme limit of the experimenter's ability, because the charge on the electron cannot be determined to greater accuracy than that with which the distance is known.

"The magnifying telescope may be improvised from a microscope, from the small telescope of a war-surplus bombsight, from a telescope gunsight, from half a binocular or from any similar instrument that provides a tube and an eyepiece. The objective lens should have a focal length on the order of 50 millimeters, depending upon the distance between the observation port and the axis of the pinhole in the upper electrode. The lens from a 35-millimeter camera will do. As an alternative, the focal length of the telescope objective may be shortened by adding an auxiliary lens. The diameter of the objective lens is unimportant. Any combination of objective and eyepiece that can resolve light reflected by a sphere .006 millimeter in diameter is adequate. The eyepiece of spanning a vertical distance of at least a millimeter on the axis of the pinhole midway between the electrodes.

"Once the telescope has been adjusted so that drops falling through the pinhole are in sharp focus, it should be clamped in position. The optical system must then be calibrated. A target ruled with accurately spaced hairlines is placed vertically on the axis of the pinhole, and the image of the lines is noted with respect to those on the reticle. The calibration must be made with the greatest possible care. Microscope slides ruled with hairlines spaced in fractions of a millimeter are available from optical-supply houses. A similar target may be made by ruling hairlines on a metal plate by means of a height gauge of the type used by toolmakers.

"Power for charging the electrodes may be derived from a conventional vacuum-tube rectifier [see circuit diagram above]. Electrolytic capacitors are not manufactured for voltages in this range, and oil capacitors rated at 1,000 volts are costly. The ripple current and load are low, however, so if one is willing to risk an occasional blowout, oil capacitors of 400-volt rating may be used. (I have had a bank of 600-volt oil capacitors across 2,000 volts for longer than I care to remember. On the average, I lose one about every two years.) The field voltage should be held as constant as possible during an experimental run. This is best accomplished by inserting a variable transformer such as a Variac in the power line. A 10,000-ohm variable resistor in series with the 100,000-ohm bleeder resistor is next best. The field voltage should be monitored constantly with a good laboratory voltmeter during a run. Do not turn off the power supply to observe drops in free fall; use the switch shown in the diagram. This switch not only cuts off the voltage but simultaneously grounds and short-circuits the electrodes, thus killing the field completely.

"A 85-millimeter slide projector makes an adequate light source. Contrast will be improved by covering the rear wall of the enclosure at the level of the space between the electrodes with a swatch of black velvet. The drops should appear as brilliant points of light against a jet-black field.

"One note of caution: Never forget that you are working with lethal voltage. Resist the temptation to grope for the field switch or other controls while squinting through the telescope. Finally, provide writing materials, a stop watch and a good barometer.

"To make a run, fill the atomizer with a light, nonvolatile oil. Do not use watch oil, which reacts chemically with lucite. Check to assure that the field voltage is off, then spray the smallest possible amount of oil into the upper chamber. (Excess oil accomplishes nothing beyond increasing the frequency of clean ups.) Then relax for 10 minutes or so. Interesting drops in free fall cover about three millimeters per minute, so they require an appreciable interval to find their way through the pinhole into the observation space. In the meantime a table may be ruled, headed like the one made by Millikan [see above]. Now light the region beneath the pinhole with the slide projector. When the telescope shows drops in the field, switch on the field voltage momentarily to eliminate drops carrying heavy charges.

"Now enter the barometric pressure and the temperature of the chamber on the table just prepared. Apply the field again and search for a drop that rises slowly. (This assumes that the telescope is equipped with an erector system. If it is not, the motion will appear reversed.) Permit the drop to rise about a millimeter above a selected graduation on the reticle. Then switch off the field and time the drop in free fall. Permit the drop to move a millimeter beyond the terminal graduation. Record the reading on the table under 'free fall.' Then switch the field on and time the return transit. If the field is heavily populated by drops, it may be difficult to keep track of a selected drop while looking away to record the transit time. This difficulty can be overcome either by learning to make entries on the table while keeping one eye on the drop, or by enlisting a helper to do the recording while you call out the observations.

"Occasionally a drop will collide with an ion and change speed. Record the transit and go right on tabulating. You can throw out the questionable entry later. Try to record 50 or more transits before shifting to another drop.

"After the times of free fall and rise have been tabulated, the constant values are recorded. These are the distance in centimeters between the electrodes, the distance of rise and fall, the field potential, the oil density ( the reciprocal of the weight, in grams, of one cubic centimeter of oil), the temperature, the barometric pressure and the viscosity of air.

"The rest is plain arithmetic. First run off the average time of free fall for all the drops observed. Note that Millikan's table lists only 11 observations of free fall. This compares favorably with the number of rise times observed for each state of charge on the drop. The figures are therefore about equally good. The rise times for each state of charge are then averaged and entered on the table. The distance of rise and fall is now divided by the average times to give the speed of the drops in centimeters per second. (On Millikan's table the average time of free fall through .145 centimeter is 34.17 seconds. This converts to a velocity of .00424 centimeter per second.) The computed value is entered as the terminal velocity (V) in the formula for the radius of the drop (r) given in the accompanying table [below]. The remaining values required by the formula are taken from the table and from a reference text such as Handbook of Chemistry and Physics.

"The radius of the drop is then computed and entered in the formula for electric charge (q), also given in the table. After inserting the other constant values required by the formula, the 3 charge is computed for each of the rise times and entered on the table under 'Calculated Charge on Drop (Electrostatic Units)'. Determine the difference between each of the tabulated charges by subtracting the lower value from the next higher for all computed tabulations. A certain minimum difference will be found. The total charge on each drop will be some integral multiple of this minimum, which should approximate 4.8 X 10^-10. Dividing the calculated charge on the drop by this minimum gives the number of charges on the drop. (The quotient must be rounded off to the nearest whole number.) The calculated charge on the drop is then divided by the number of charges. This quotient is entered in the final column ('Magnitude of Individual Charge') and compared with the most recent determination for the charge on the electron: 4.8029 +/- .0001 X 10^-10 electrostatic units."

Some appreciation of the far-reaching consequences of this experiment can be gained by considering its influence upon the theory of the photoelectric effect. All substances emit electrons under the influence of light. By placing a negative electrode close to the substance and connecting a battery between the two, a voltage can be found that is just sufficient to stop outward-bound electrons. It turns out that the stopping voltage depends solely on the color of the light and not at all on its intensity. Einstein explained this in 1905 by supposing that all the ejected particles carry an identical charge, which when multiplied by the stopping voltage just equals the product of a certain quantity h (which Max Planck had derived to explain the radiation of energy from a "black body") and the frequency of the light minus the amount of energy required by the particle to break free of the substance. This theory made no sense to most physicists of the day, because it required the light to be radiated in chunks of a size determined by the constant h, which Planck apparently had pulled out of thin air.

"At the time it was made," wrote Millikan, "this prediction [by Einstein] was as bold as the hypothesis which suggested it, for at that time there were available no experiments whatever for determining anything about how the potential necessary to stop the discharge of the negative electrons varied with the frequency of the light or whether the quantity h to which Planck had already assigned a numerical value appeared a all in connection with photoelectric discharge. We are confronted, however by the astonishing situation that after ten years of work at the Ryerson Laboratory this equation of Einstein's seems to us to predict accurately all of the fact which have been observed!"

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