An automatic plotter for magnetic hysteresis loops

The instrument measures the hysteresis loops of magnetic materials and plots the curves directly by pen on paper. The measurements are made under virtually zero-frequency conditions, the period of the cycle being nearly one minute. The magnetic flux is detected by a search coil on the sample, the e.m.f. being amplified and integrated by avariable-speed motor on the plotting table, whereby the displacement of the pen is a measure of the magnetic induction. The instrument is calibrated in terms of voltage and time, which offers good absolute accuracy. Comparative experiments against afixed mutual inductance and a Hibbert magnetic standard show agreement within about 1%. The equipment is intended to give measurements equivalent to those obtained with a ballistic galvanometer, but with greater convenience; a comparison with a wide rangeof samples shows agreement within ±1%, with certain exceptions. In Appendices are given (a) a circuit of a sensitive servo system used in plotting the magnetizing current, incorporating a magneticinverter; (b) considerations for the design of a galvanometer to berelatively unaffected by mechanical disturbances; and (c) the design of an optical system for a galvanometer-photocell amplifier giving maximum optical efficiency.