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I. The mass of the electric carrier in metals. II. The lack of effect of a magnetic field on the dielectric constant of HCl and NO


Mott-Smith, Lewis M. (1926) I. The mass of the electric carrier in metals. II. The lack of effect of a magnetic field on the dielectric constant of HCl and NO. Dissertation (Ph.D.), California Institute of Technology.


The Berkeley Experiments: The production of an electromotive force in an accelerated metallic conductor, due to the inertia of the electrons in the conductor, was first demonstrated by the work of Tolman and Stewart at Berkeley, who measured the pulse of electric current produced by suddenly stopping a coil of wire rotating around its axis. Their experiments included work on a number of different coils of copper, silver and aluminum wire. The pulse of current was always found to be in the direction which would be predicted on the basis of a mobile negative electron for the carrier of electricity in these metals, and the mass of the carrier as calculated from the magnitude of the pulse came out for the three metals tested about 15% higher than the known mass of an electron in free space.

The most serious chance of error in the Berkeley experiments would seem to be the possibility of electromotive forces produced between the wire and its insulation at the time of stopping of the coil. Some evidence of such disturbing effects would seem to be given by the work of Tolman and Stewart, since they found that their results were more concordant the more carefully they wound their coils and the more care they gave to the impregnation of the coil with paraffin or shellac. Further evidence as to such possible misbehavior of the insulating material will be given in the present article.

The Washington Experiments: The above chance of error was eliminated in the later experiments of Tolman, Karrer and Guernsey made at Washington, who oscillated a cylinder of copper around its axis and determined the electromotive forces produced in it by surrounding it with a secondary coil of many turns of fine wire which was connected through an amplifier with a vibration galvanometer. These experiments also seemed to demonstrate an electromotive force due to the inertia of the electrons, and the average value for the mass of the carrier came out from these measurements about 8% lower than the mass of the electron in free space.

In these Washington experiments only the effective amplitude of the alternating current produced in the secondary was measured and this compared with the current produced in the secondary by a suitable method of calibration. Hence since the phase of the current was not known, it is impossible to conclude from these experiments, as from the Berkeley work, that the direction of the effect is necessarily that which would be predicted on the basis of a negative charge for the mobile carrier.

The Washington experiments also suffered from an uncertainty as to the best method of correcting for the accidental electromotive forces always present in the coil, which produced at all times a varying deflection of the galvanometer beam, even though the apparatus was not running. The method adopted was to measure the width of the galvanometer beam when the apparatus was stationary (zero effect) just before or after a series of measurements and then correct simply by subtraction. It is evident, however, that a method of balancing out that part of the electromotive force of real interest would be superior.

Finally the time available for the Washington experiments made it impossible to carry out a complete study of the effect of the earth’s magnetic field. To eliminate effects from this source, the oscillating cylinder was set with its axis as nearly parallel as possible to the direction of the earth’s field, and the axis of the coil was then aligned parallel with the axis of the cylinder. It was believed that this arrangement would eliminate accidental effects due to motion in the earth’s field, provided exact parallelism could be attained, and indeed the work to be described in the present article substantiates this point of view. Time did not permit, however, a satisfactory determination of the magnitude of the error actually introduced by the impossibility of obtaining absolute parallelism. Preliminary attempts were made to neutralize the earth’s field with large Helmholtz coils, and changes in the magnitude of the effect were observed to accompany changes in the D.C. current in these coils. At the time, this effect was ascribed to eddy currents in the cylinder produced by lack of homogeneity in the field introduced by compensating coils. The true nature of this effect, however, has now been found, and will be made apparent in the present article.

The General Nature of the Pasadena Experiments: In the present experiments, which were made at Pasadena, we have again oscillated the copper cylinder parallel to its axis and surrounded it by a secondary coil of many turns of fine wire, but have overcome the above three difficulties encountered in the work at Washington by methods that will be described below.

In order to determine the phase of the effect we have balanced the alternating electromotive force in the coil against an alternating electromotive force produced by an earth inductor rotating in synchronism with the oscillation of the cylinder, arrangements being made so that we could adjust the amplitude and phase of the balancing electromotive force.

This method of measuring the effect also eliminates uncertainty as to the right way of correcting for the "zero effect" which keeps the galvanometer oscillating to some extent all the time, even when the apparatus is not running, since we only balance out the electromotive force of actual interest. Under favorable conditions there was almost no difference between the widening and "kicks" in the band of light from the galvanometer when the balance had been made with the apparatus running and the behavior of the band when the apparatus was stationary.

Finally, we have made an elaborate study of the effect of the earth’s field on the magnitude and phase of the effect, by changing the alignment of the cylinder with respect to the earth’s field and the alignment of the coil with respect to the cylinder, and by rotating the coil around its axis. Indeed, the investigation of such effects forms a considerable portion of the work to be described in this article. In a general way it may be said that the correct magnitude of the effect will be obtained provided the cylinder is exactly parallel to the earth’s field or provided the (electrical) axis of the coil is parallel to the cylinder. If both alignments are made parallel the results are of course that much better. This explains the success of the Washington experiments, since the attempt was there made to have both alignments parallel.

Item Type:Thesis (Dissertation (Ph.D.))
Degree Grantor:California Institute of Technology
Division:Physics, Mathematics and Astronomy
Major Option:Physics
Thesis Availability:Public (worldwide access)
Research Advisor(s):
  • Unknown, Unknown
Thesis Committee:
  • Unknown, Unknown
Defense Date:1 January 1926
Record Number:CaltechETD:etd-11112004-135749
Persistent URL:
Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:4507
Deposited By: Imported from ETD-db
Deposited On:11 Nov 2004
Last Modified:26 Dec 2012 03:09

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