Galvanometer Constant.
Assume a galvanometer with a very short needle and so placed with
respect to its coils that the magnetic field produced by a current
circulating in them is sensibly uniform in the neighborhood of the
needle, with its lines of force at right angles thereto. The field is
proportional to the current i, so that it may be denoted by G i. Then G
is the galvanometer constant. If now the angle of deflection of the
needle is ? against the earth's field H, M being the magnetic moment of
the needle we have G i M cos ? = H M sin ? or i = (H/G)* tan ?. H/G is
the reduction factor; variable as H varies for different places.
For a tangent galvanometer the constant G is equal to 2*PI*(n/a), in
which n denotes the number of turns of wire, and a denotes the radius of
the circle.
Galvanometer, Differential.
A galvanometer in which the needle is acted on by two coils wound in
opposition, each of equal deflecting action and of equal resistance. If
a current is divided between two branches or parallel conductors, each
including one of the coils, when the needle points to zero the
resistances of the two branches will bc equal. In the cut, C C'
represent the coils, and A and B the two leads into which the circuit, P
Q, is divided.