6
KINETIC ENERGY OF ELECTRICITY IN MOTION [1]
thus obtained. With the greatest strength, which still per-
mitted permanently of such an adjustment, a single extra-
current from the two branches, when traversed in opposite
directions, only moved the galvanometer needle through a
fraction of a scale division, whilst the mere approach of the
hand to one of the mercury cups, or the radiation of a distant
gas flame falling on the spirals, sufficed to produce a deflection
of more than 100 scale divisions. Hence I attempted to
make use of very strong currents by allowing them to pass
for a very short time only through the bridge, which was
adjusted by using a weak current. But the electromotive
forces generated momentarily in the bridge by the heating
effects of the current were found to be of the same order of
magnitude as the extra-currents to be observed, so that it
was impossible to get results of any value. These experi-
ments only showed that at any rate there was no consider-
able deviation from the laws of the dynamical theory of
induction.
On this account, in order to obtain measurable deflections
with weaker currents, I passed a considerable number of extra-
currents through the galvanometer at each passage of the
needle through its position of rest. For this purpose the
current was at the right moment rapidly reversed twenty
times in succession outside the bridge, and at the same time
the galvanometer was commutated between every two successive
reversals of the current. In order to avoid any considerable
damping, after the bridge had been once adjusted the galvan-
ometer circuit remained open generally, and was only put in
circuit with the rest of the combination during the time
needed to generate the extra-currents.
The operations described were carried out by means of a
special commutator and occupied about two seconds, an in-
terval of time which was sufficiently long to allow of all the
extra-currents being fully developed, and which also proved to
be sufficiently small in comparison with the time of swing of
the needle.
This method possessed several advantages. In the first
place accurately measurable effects could be produced even
with very weak, and therefore also very constant, inducing
currents. In all of the following experiments the external