Signedon instrument's face: AYRE & PERRY'S / DIRECT READING / SPRING AMMETER / PATENTED 1883 / LATIMER CLARK, MUIRHEAD & Co. L'd. / WESTMINSTER
FunctionAyrton and Perry's direct reading spring ammeter was one of the first ammeters that gave a reading directly in amperes. The instrument has a coil inside the cylinder, which turns into an electromagnet when current is circulating. This electromagnet pulls an iron core that is attached to a spring. As a result a needle attached to the spring turns in proportion to the distance that the iron is displaced.
Related WorksThis instrument is described in:
Robert Bud and Deborah Jean Warner, eds., Instruments of Science: an historical encyclopedia. New York: Science Museum, London and National Museum of American History, Smithsonian Institution, in association with Garland Pub., 1998.