Experimental demonstration of the equivalence of a mechanically oscillated electrostatic charge to an alternating current
Method of testing equivalence of a mechanically osciallted charge to an alternating current.—The apparatus consisted of a cylinder approximately 4 inches in diameter by 9 inches long, constructed of insulating material, but coated on the out-side with a thin sheet of copper foil which could be charged to potentials which varied from 1400 to 2600 volts. The cylinder was arranged to oscillate around its axis at frequencies in the neighborhood of 20 cycles per second through an amplitude of something over 180°. It was surrounded by an "effect coil," having 260,772 turns of fine wire, which was connected through a three-stage amplifier with a tuned vibration galvanometer for detecting the electromotive force which would presumably be induced by the oscillation of the charged cylinder. The "effect coil" was shielded by a grounded covering of sheet copper, the inner portion of this covering and the coating of copper foil on the cylinder thus forming the two plates of an electrostatic condenser. Connected in series with the "effect coil" was a similar "compensating coil" having the same number of turns but connected in opposition so as to neutralize the effect of external magnetic disturbances. The electromotive force induced in the "effect coil" was measured by balancing out with the help of an auxiliary current flowing in a current sheet in such a way as to induce an opposing electromotive force in the "compensating coil," this auxiliary current having the same frequency as the electromotive force to be balanced and being adjustable as to phase and amplitude. The apparatus was calibrated by replacing the charged oscillating cylinder by a current sheet of known dimensions and carrying a known current. Accidental effects coming from a number of sources were studied and as far as possible reduced or eliminated. Results of the test and conclusions.—A total of 64 effect runs were made, together with the necessary blank runs for correcting for the small residual effect. The runs were made in 16 different groups of 4 runs each under a given set of conditions. The changes in conditions consisted in changes in the sign and magnitude of the potential applied to the cylinder, changes in the method of connecting the "effect coil" and "compensating coil" to the grid and filament of the amplifier, changes in the neutral position around which the cylinder was oscillated, and change to a different frequency of oscillation. The phase of the effect obtained depended on the conditions of operation in the way predicted by theory. It was changed through 180° by a reversal of the sign of the applied potential, depended on the connections used and on the frequency of oscillation in the manner predicted from the calibration runs, and was independent of the neutral position around which the cylinder was oscillated. The magnitude of the effect obtained was also in agreement with theory. It was closely proportional to the potential applied to the cylinder and to the velocity of motion of the cylinder, and had very approximately the absolute value predicted from the measured capacity of the cylinder, the applied potential and the velocity of motion. The result of the experiment is to show that a mechanically oscillated electrostatic charge of electricity surrounds itself with an alternating magnetic field, accompanied at right angles by an alternating electric field capable of producing an alternating electromotive force in a suitably placed secondary. It further shows that at right angles to both the magnetic and electric fields there must be a Poynting-vector field corresponding to the observed transmission of energy to the secondary, and that these quantities all have the magnitude and time dependence predicted by electromagnetic theory. The present experiment must be regarded as testing a more extended portion of the fundamental basis of electromagnetic theory than the original Rowland experiment, and as giving a more clear-cut demonstration of the equivalence of a mechanically oscillated charge of electricity to an ordinary alternating current than have previous experiments.
©1929 The American Physical Society. Received 30 July 1929. We desire to express our appreciation to the California Institute for the support which it gave to this expensive investigation, and to give our thanks to Mr. Julius Pearson and his assistants for their skill in the construction of apparatus.