14
I
KINETIC ENERGY OF ELECTRICITY IN MOTION [1]
distance between successive hooks, so that a reversal of the
galvanometer occurs between every two reversals of the cur-
rent. While the needle is completing its swing after the
induction impact, the hooks are symmetrically situated with
respect to the vessel C, so that one hook is above the space
between the two halves of the middle mercury cup, and the
neighbouring ones are right and left at the sides of the cups;
the connection of the galvanometer with the bridge is then
broken. As soon as the needle reaches its position of rest the
disc is turned by hand and after a whole turn is stopped by
a simple catch, so that then the commutator performs the
above operations.
It may be mentioned that generally the wires of the
bridge, wherever possible, were soldered directly to each other;
binding-screws and mercury-cups were only used where con-
nections had to be broken and remade repeatedly.
EXPERIMENTS WITH DOUBLE-WOUND SPIRALS.
I now come to the individual experiments, and first to
those with double-wound spirals. I had at my disposal two
spirals, exactly similar and very carefully wound, whose length
was 73.9 mm. and whose external and internal diameters were
respectively 83.6 and 67.3 mm. They consisted of eight
layers, each with sixty-eight turns. The total length of wire
was found by comparison of its resistance with that of the
outer layer to be 130,032 mm. The diameter of the wire
was 0.93 mm., the total resistance about 3.1 Siemens units.
As the spirals were exactly alike, they were used together and
put in the diagonally opposite arms of the bridge. The extra-
currents produced by them were then added together in the
bridge.
According to the above explanations the inductive effects
of two inductances P and P' were to be observed whilst the
resistance of the circuit remained unchanged. The inductance
P was that of the spiral when the current in its two branches
flowed in opposite directions. To obtain a second inductance
P' a branch of one spiral was thrown out of circuit and re-