The instrument presented here, designed by Latimer Clark, is a differential galvanometer with a mobile needle that was used for many years in telegraphy to measure the resistance of the lines.
It consists of two identical, coaxial coils having the same resistance, arranged symmetrically to the needle, with their frames in contact; thus the same current flowing through the two coils in opposite directions produces no movement of the needle. To measure a resistance it is sufficient to introduce into the circuit of one of the two coils the unknown resistance and in the circuit of the other a known resistance that brings the galvanometer needle to zero, then the known resistance is the same as the unknown one.
As specified by Clark himself in a treatise on electrical measurements used by the French telegraph service, the instrument was equipped with a series of resistances from 1 to 10,000 ohms as in the box with decades which are supplied with Wheatstone bridges.
The instrument, with its terminal board and connecting wires, is mounted on a rectangular bakelite board (width 13.3, depth 18 cm) which is attached to a polished wooden base with the sides well-lathed and slightly protruding. ...continues