British Post Office type of Differential Galvanometer
Wound with two separate coils of 100 ohms each, with short circuit blocks. Used extensively on all 'balanced' circuits such as Duplex, Quadruplex, and Wheatstone. Automatic for obtaining and preserving a true equality between the 'real' and 'artificial' lines. The active needle is kept magnetized by the proximity of two bar magnets. The terminals of the two coils are on the back. When used differentially the current is made to pass through the two coils in opposite directions, so that the magnetic effect on the needle is due to the difference in the strength of currents. When the currents are equal the needle is not affected.