IV
141
KINETIC ENERGY OF ELECTRICITY IN MOTION [II]
has ceased. This method of calculation could only lay claim
to accuracy if the rotation were uniform and the permanent
deflection small; but the disturbances were too various and
the deflections too irregular to permit of fuller discussion.
The first experiments already showed that if there was
any effect of inertia, it did not much exceed the errors un-
avoidably introduced by disturbing causes. In order to detect
such an effect, and to find as small as possible a value of its
upper limit, I took a set of eight observations together, in
which the direction of rotation was changed between every
two observations: the connection to the galvanometer was
reversed every other observation, and the current in the plate
was reversed between the first four and the last four observa-
tions. Such a set of eight observations I call an experiment.
By suitably combining the observations it would be possible
to calculate the mean effect of the various disturbing causes
for each experiment. For the deflections must include, and
we should be able to eliminate from them :—
1. A part which changes sign only when the connection
with the galvanometer is reversed, but not when the direction
of rotation or the connection to the battery is changed. It
could only be due to an electromotive force generated by the
rapid rotation at the point of contact of the galvanometer
circuit. In so far as this force was thermoelectric the cor-
responding deflection must have been permanent.
2. A part whose sign depended on the direction of the
galvanometer and battery connections, but not on the direction
of rotation. This could be due to various causes:
(a) The straining of the plate by the considerable centri-
fugal force, whose effect could only appear in the momentary
deflection.
(b) An uniform change of temperature of the whole plate
owing to rotation, whose effect would be felt in the permanent
deflection.
(c) A change in the ratios of the resistances AC/BC and
AD/BD during the experiment, due to external causes. In
fact the resting-point of the needle changed slowly even when
there was no rotation, but continuously and so much that the
error produced was of the order of the others.
felt in the permanent deflection.
The effect was