The Standard Electrical Dictionary

1. 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.