of the Movable-coil Galvanometer. 79 
pointer, p q, is situated in the same plane as the scale; the 
ends are flattened and provided with a fine slit, which serves 
as an index for reading the deflections, the bottom of 
the needle-box being blackened. The reading can thus be 
taken without parallax, and therefore very accurately. The 
magnetic needle has a biconical shape, which entirely prevents 
the shifting of the magnetic axis from its original position, 
as was sometimes found to be the case with the old broad 
needles. Adjustments are provided by which the cocoon- 
fibre, /, serving to suspend the needle, can be raised or 
lowered, as well as accurately centred. 
The Damping of the Oscillations. 
Numerous experiments were undertaken to ascertain a con- 
venient method for damping the oscillations of the needle, and 
to arrive, if possible, at a perfectly aperiodical movement. 
After trying large masses of copper placed in the immediate 
neighbourhood of the swinging magnet, as well as liquid 
damping, without decided success, air-damping was resorted 
to, and finally adopted. It will be remembered that Sir Wil- 
liam Thomson used air-damping for the light-mirror of his 
dead-beat galvanometer, and Prof. Topler* for other galvano- 
metric apparatus. In our case the air-chamber consists of a 
shallow cylindrical box, about 8 centim. in diameter, 1^ centim. 
high, provided with two radial partitions which can be slid in 
or out; the axle of the needle, passing through the centre of 
this box, carries a light and closely fitting vane. By sliding 
the partitions more or less into the box various degrees of 
damping can be obtained; and if they are right in, the motion 
is practically dead-beat. 
The Scales. 
Declination-scale. — This scale, engraved on a horizontal ring, 
was formerly divided into degrees, as usually done ; but now 
one semicircle is provided with divisions corresponding to the 
natural tangents. The interval between each two divisions 
must of course vary for different pari 
arranged as follows: — 
Table I. 
Values of tangent. 
Interval. 
to 1 
o-oi 
1 „ 2 
0-02 
2 „ 3 
0-05 
3 „5 
o-io 
Repert.f. exp. Phys. ix. p. 259 (1873). 
H 2 
