214
XI
HOT-WIRE AMMETER
to be an elastic rather than a thermal after-effect, and is not
greater than in other instruments in which forces are measured
by the elasticity of wires. After a few minutes, at the outside,
the image returns to the original position of rest.
4. The following data indicate the sensitiveness of the
instrument. It was included in a circuit containing a Daniell
cell and a resistance of r Siemens units. In the following
table a denotes the deflection in scale-divisions, and b the
square root of this deflection multiplied by the total resistance
of the circuit (consisting of r S.U. together with 0.85 S.U. for
the instrument, and 0.77 S.U. for the Daniell cell) and divided
by 10.
r =
b
=
100 50 30 20
3
a= 0.25 0.9 2.2 4.9 16.9 52.1 106.8 173.8
4.94 4.89 4.68 4.77
4.78 4.77 4.77
10
5
2
4.77
The numbers in the third row, excepting those corre-
sponding to the smallest deflections, are all equal: this shows
that the deflections are proportional to the square of the
current, and that the instrument is well adapted for measure-
ments. The current sent by 1 Daniell through 100 to 150 S.U.
can be easily detected: currents sent by 1 Daniell through
30 S.U. and, by means of shunts, all stronger currents, can be
measured.
5. When currents alternating a few hundred times per
second are sent through the instrument, there arises a difficulty
which is due to the small period of vibration of the mirror.
The wire absorbs and emits heat very rapidly, and the mirror
oscillates in accordance, following every impulse. In itself
this is an advantage: but as the eye cannot follow the
oscillations, the image of the scale becomes indistinct and the
mean deflection cannot be accurately read off. This difficulty
is much reduced by using the objective instead of the subjective
method of observation; the scale then remains at rest, and
although the spot of light oscillates backward and forward, its
mean position can be accurately determined. Furthermore,
without diminishing the sensitiveness, the period of vibration
can be increased at will by increasing the moment of inertia
about the axis.
It appeared that the sensitiveness of the instrument was