Fig. 185. TANGENT GALVANOMETER.
Galvanometer, Tangent.
A galvanometer in which the tangents of the angles of deflection are
proportional to the currents producing such deflections.
For this law to apply the instrument in general must fulfill the
following conditions:
(1) The needle must be controlled by a uniform magnetic field such as
that of the earth;
(2) the diameter of the coil must be large compared to the length of the
needle;
(3) the centre of suspension of the needle must be at the centre of the
coil;
(4) the magnetic axis of the needle must lie in the plane of the coil
when no current is passing.
If a single current strength is to be measured the best results will be
attained when the deflection is 45°; in comparing two currents the best
results will be attained when the deflections as nearly as possible are
at equal distances on both sides of 45°.
The needle should not exceed in length one-tenth the diameter of the
coil.
For very small deflections any galvanometer follows the law of
tangential deflection.
As for very small deflections the tangents are practically equal to the
arcs subtended, for such deflections the currents are proportional to
the deflections they produce.
The sensibility is directly proportional to the number of convolutions
of wire and inversely proportional to their diameter.
The tangent law is most accurately fulfilled when the depth of the coil
in the radial direction is to the breadth in the axial direction as
squareRoot(3):squareRoot(2), or about as 11:9.
Galvanometer, Torsion.
A galvanometer whose needle is suspended by a long filament or by a
thread and spiral spring against whose force of torsion the movements of
the needle are produced. The current strength is determined by bringing
the needle back to its position of rest by turning a hand-button or
other arrangement. The angle through which this is turned gives the
angle of torsion. From this the current strength is calculated on the
general basis that it is proportional to the angle of torsion.
Fig. 186. TORSION GALVANOMETER.