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.