Galvanometer, Volt and Ampere-meter

A galvanometer of Sir William Thomson's invention embodying the tangent principle, and having its sensibility adjustable by moving the magnetic needle horizontally along a scale (the "meter") towards or away from the coil. A curved magnet is used to adjust the control. The leads are twisted to prevent induction. The instrument is made with a high resistance coil for voltage determinations, and with a low resistance coil for amperage determinations. At one end of a long base board a vertical coil with its plane at right angles to the axis of the board is mounted. A scale (the "meter" of the name) runs down the centre of the board. A groove also runs down the centre. The magnetic needle is contained in a quadrant-shaped gla -covered box which slides up and down the groove. A number of short parallel needles mounted together, with an aluminum pointer are used. IMG:8956969607833025625_274F188.jpg: Fig. 188. SIR WILLIAM THOMSON'S 275   STANDARD ELECTRICAL DICTIONARY. In the cut P is the base board, M is a gla covered case containing the magnetic needle, and sliding along the base board, being guided by the central groove, C, is the coil. Between the coil and the needle is the arched or bent controlling magnet. The long twisted connecting wires are seen on the right hand.