This is a reproduction, made by Tisley and Spiller in 1873-1877 of an early type of tangent galvanometer. Tangent galvanometers measure current using Hans Christian oersted's 1820 discovery of the magnetic effect produced by an electric current. The galvanometer is turned until the coil points the same way as the compas needle. Current pasing through the coil then creates a magnetism at right angles to the magneticism of the Earth, and the needle swings to a new angle which shows the strength of the current.
© Science Museum / Science & Society Picture Library