212
XI
HOT-WIRE AMMETER
presence or absence of currents alternating more rapidly than
10,000 times per second, the dynamometer could tell us
nothing; for its introduction into the circuit would prevent
the establishment of such currents. For example, it could
not be used for investigating the discharge of a Leyden jar
through a short metallic circuit.
In pursuing an investigation¹ which depended upon
detecting unusually rapid alternating currents, I found it
necessary to have a fairly delicate instrument of small resist-
ance and negligible self-inductance; and it occurred to me to
use the heating effect of the current in thin metallic wires as a
means of detecting it. The attempt succeeded much better
than was to be expected, and I may here be allowed to
describe the simple instrument which I used. For a given
current it certainly gives a much smaller deflection than the
A
usual dynamometers. But it is much
more delicate than any instrument of
comparable resistance, its self-induct-
ance is negligible, and it is as easily
handled as any other instrument
which gives equally accurate results.
The apparatus is shown in Fig.
24. The essential part of it consists
of a very thin silver wire, 80 mm.
long and 0.06 mm. in diameter,
stretched between the screws A and
B; the wire does not run right
across from the one screw to the
other, but is attached by a little
solder to the vertical steel wire ab
and twisted round this, as shown in
Fig. 24, b. The steel wire ab has a
diameter of 0.8 mm., and is as smooth
and round as possible; the twisting
of the silver wire can easily be
managed by first stretching it loosely and then turning the
steel wire in the direction of the arrow. The silver wire
being now well stretched, ab is held in position by the torsion
which it produces in the thinner steel wires ac and bd; these
1 See XIII. p. 224.

FIG. 24.
I