138
IV
KINETIC ENERGY OF ELECTRICITY IN MOTION [II]
from the direction of the meridian. The consequence of this
tendency will be a difference of potential between the points
C and D, and a current through the galvanometer. This
current must be reversed when the direction of rotation is
reversed; when the rotation is clockwise and the current flows
in the plate from A to B, then the current through the galvan-
ometer outside the plate must flow from D to C, as shown by
the arrows.
An action of the kind mentioned must occur, whatever be
the nature of the electric current, provided only that with it
a motion of an inert mass is connected, which changes its
direction when the current is reversed. The difficulty of the
experiment consists in preparing four connections, sufficiently
certain and steady, even with rapid rotations; this difficulty
I have overcome to such an extent that one of the most deli-
cate galvanometers could be used when the velocity was 30
turns per second, and the difference of potential between A
and B was that of 1 Daniell. No deflection of the needle
could be detected which would indicate the existence of electric
inertia. Basing my calculations on Weber's hypothesis, I am
able from my experiments to infer by the method given below
that μ, the kinetic energy of a current of magnetic strength 1
in a cubic millimetre of a silver conductor, cannot greatly
exceed 0.00002 mg. mm.²/sec².
As regards the method of experimenting I may mention
the following. The metal plate used was the silvering of a
glass plate, produced by Liebig's process. Its form is shown.
in Fig. 17; the distance AB was about 45 mm., the distance
CD 25 nm. The leads were soldered to small platinum
plates, and these were pressed into contact with the silvering
by small screws penetrating the glass plate; a layer of gold-
leaf was introduced between the silvering and the plates, so as
to produce a more uniform contact. The electrical resistance
was at first 5-4 Siemens units in the direction AB, and 3.5
Siemens units in the direction CD. From some unexplained
causes these resistances diminished in time, and after some
weeks were found to be 4.8 and 3.1 Siemens units respectively.
From the ratio of these resistances and from special experi-
ments, it followed that the resistances of the contacts at the
leads did not amount to any appreciable fraction of the whole