Ayrton and Perry ammeters, late 19th century.
© Science Museum / Science & Society Picture Library
Overall and cros section views, of ammeters developed by William Edward Ayrton (1847-1908) and John Perry (1850-1920). Most meters measure electricity by changing a little of the electrical energy into magnetism, and making the resulting magnetic force move a needle on a dial. This meter was one of the first to measure current and voltage reliably.