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I
KINETIC ENERGY OF ELECTRICITY IN MOTION [1]
them of sufficient constancy. Hence I employed wires of
German-silver, which were passed through glass tubes and
surrounded by distilled water, so as to guard against changes
of temperature. These were so arranged that those belonging
to different branches and traversed in opposite directions lay
side by side. The values of the inductances still remaining
were small and could be allowed for with sufficient accuracy
in the calculation. Since the German-silver wires were very
thin there was a danger that they might, when the current
was reversed, be subjected to small but sudden changes of
temperature. Such changes would, at the instant when the
current was started, have disturbed the balance of the bridge,
and so would have produced an increase or decrease of the
extra-current very difficult to estimate. Therefore, in a last
series of experiments I employed rods of Bunsen gas-carbon,
5 mm. in diameter, such as are used for electric lighting.
The
3. The strength of the inducing current was measured
outside the bridge; the tangent galvanometer used consisted
of a single copper ring 213.2 mm. in diameter, at the centre
of which a needle about 25 mm. long was suspended by a
single silk fibre. In order to damp its vibrations as quickly
as possible it was placed in a vessel of distilled water.
readings were taken by telescope and scale; the distance of the
latter from the needle was 1295 mm., and one scale-division
corresponded to a current of 0.01218 in absolute electro-
magnetic units. The measurements were always made by
observing a deflection to the right, then one to the left, and
then again one to the right. The result is correct to 10 of
its value.
The extra-current was measured by a Meyerstein galvan-
ometer of very low resistance, such as is used for measurements
with the earth-inductor. The pair of needles was suspended
astatically by twelve fibres of cocoon silk; the time of swing
was 27.66 seconds. The galvanometer was set up on an
isolated stone pillar, 2905 mm. from the scale and telescope,
and about the same distance from the bridge, and was connected
with the latter by thick parallel copper wires.
4. The commutator, at each passage of the needle through
its position of rest, had to perform the following operations
quickly one after the other :-
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