Further Experiments on the Mass of the Electric Carrier in Metals

Abstract

Inertia of the electric carrier in metals.---Although Tolman and Stewart had apparently demonstrated that the carriers in metals have approximately the same ratios of mass to charge as an electron, it was desirable to verify this result and if possible obtain a more accurate value for $\frac{m}{e}$ by using an entirely different method. An oscillating apparatus was constructed, consisting of a copper cylinder $9\frac{1}{8}$\ensuremath{''} long, 4\ensuremath{''} outside diameter and 3\ensuremath{''} inside diameter, attached to a brass torsion rod in such a manner that it could be oscillated about its axis with a frequency of about 20 cycles per second. Surrounding this copper cylinder was a coil, containing about 60 miles of No. 38 copper wire, which acted as the secondary of a transformer, and was connected to a vibration galvanometer through a specially designed three-stage amplifier. The tendency of the electrons in the oscillating copper cylinder to lag behind because of their inertia leads to an electromotive force, the effects of which were finally measured by the deflection of the vibration galvanometer, tuned to the frequency of the mechanical oscillations. This galvanometer deflection was then compared with that produced by the known electromotive force accompanying a transverse oscillation of the cylinder across the earth's magnetic field. The apparatus was mounted on a massive concrete pier in a special location 150 yards from the nearest electrical circuits, was constructed without the use of magnetic materials, and was driven by compressed air. The axis of the oscillating cylinder was set parallel to the earth's magnetic field to reduce accidental effects. The apparatus avoids the direct electric connections between moving and stationary parts, and the sudden stopping of a coil of wire, with the attendant chance of buckling and slipping of the wire, which were present in the apparatus used by Tolman and Stewart. The fact that the vibration galvanometer will respond only to the frequency of the desired effect is also important in eliminating accidental effects. Mass of carrier in copper. The average of 86 determinations of $\frac{m}{e}$ is 5.2 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}8}$. However since the means of the first 42 determinations and of the last 44, obtained with the cylinder earthed only through the concrete pier and specially earthed, respectively, are 5.97 and 4.35, and since the corrections for zero amplitude were large, these preliminary results are not regarded as demonstrating a difference between the ratio $\frac{m}{e}$ of the carrier in copper and of an election in free space, 5.66 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}8}$.