early 20th century antique american industrial ornamental cast iron surface mount commercial building sub-basement switchboard ammeter encased gauge designed, patented and fabricated by the weston electrical instrument company, newark, nj. the circular-shaped antiquated electrical measuring instrument or gauge has been brushed down to bare metal and sealed with a clear coat lacquer. the face plate retains the original clear glass panel protecting the delicate pointer and painted metal scale. the medium-sized ammeter was originally mounted flush against a slate panel switchboard. ammeters are measuring instruments used to measure the electric current in a circuit. electric currents are measured in amperes, hence the name. early ammeters were laboratory instruments which relied on the earth's magnetic field for operation. by the late 19th century, improved instruments were designed which could be mounted in any position and allowed accurate measurements in electric power systems. the weston ammeter is "moving iron" type, which uses a piece of iron that moves when acted upon by the electromagnetic force of a fixed coil of wire (mounted behind the detachable faceplate). the iron element consists of a moving vane attached to pointer and fixed vane surrounded by the coil.