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 pa ing. 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 po ible 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 square Root(3):square Root(2), or about as 11:9.