SM006     Tangent Galvanometer     circa 1935     Electricity

  A tangent galvanometer for measuring electric current consists of a magnetic needle suspended at the centre of a vertical ring through which a current flows, the plane of the ring coinciding with the magnetic meridian.   The earth’s magnetic field and the magnetic field due to the current through the coil are at right angles and so a resultant couple acts on the magnetic needle such that a deflection, q results.   If the current is I, the earth’s field is H, then it can be shown that:

where

n being the number of turns in the coil

r being the radius of the coil .

So

  The tangent galvanometer can measure large currents where n is small and small currents when n is large (about 500 turns).   If r is made smaller, the sensitivity is higher but for a uniform magnetic field from the coil, its diameter must be about 12 times the length of the magnet.