IV
139
KINETIC ENERGY OF ELECTRICITY IN MOTION [II]
resistance. The system was adjusted to bring the needle to
zero by scraping off the silver at various points of the edge;
but as a sufficiently accurate adjustment from various causes
could not be permanently obtained, shunts
of several hundred Siemens units resistance
were introduced between A and C and
between C and B, and by their adjustment
the needle could always be brought to zero,
in so far as that seemed desirable.
FIG. 17.
The glass plate was fastened to a brass
disc so as to permit of a rapid rotation;
the silvered surface faced the disc, and
was only separated from it by the thin-
nest possible air film. The disc itself was
at the end of a horizontal steel spindle,
which was set in two bearings in such a way that its two
ends were free. The connection to the galvanometer was
made at the glass plate itself; that to the battery, which
supplied the current, at the other end of the spindle; the
connections to the points A and B were formed by the spindle
itself and by a wire lying in a canal bored through the spindle.
The arrangement by which the last connection was effected
between the moving and fixed parts is shown in Fig. 18. A
fine platinum wire passes through a piece of glass tube drawn
out to a very fine point and very exactly centred. A second
platinum wire is wound round the tube; and the latter,
together with the wires, passes through one vessel of mercury
and enters a second in such a way that the first-mentioned
wire rotates in the mercury of the last vessel, and the second
wire in the mercury of the first vessel. The glass tube was
ele
FIG. 18.
fastened at one end of the
spindle by sealing-wax to
the glass plate; at the
other end to the spindle
itself. As the diameter of
the windings of the wire B was only about mm., the
platinum moved relatively to the surrounding mercury at
a speed of only 160 mm/sec, even with a velocity of rota-
tion of 100 revolutions per second. The result was good,
for even with the latter velocity there was no appreciable