A Magnetic Curve Tracer

Y ) R accurate determinations of hysteresis loops and initial magnetization curves of magnetic specimens, a laborious routine involving the use of a ballistic galvanometer is usually necessary. This article describes an apparatus by means of which these curves may be obtained photographically with quantitative accuracy. Attempts to devise such a scheme have previously been made. Ewing 1 describes one which was used with short, thick specimens in a magnetic yolk. Fleming 2 invented a device, the Campograph, which made use of a magnetometer and had the advantage of making possible the use of long, thin, specimens, thus reducing eddy current and demagnetization effects. J. B. Johnson 3 describes the most recently published design, embodying a vacuum tube amplifier and a Braun tube oscillograph. This hysteresigraph is used with frequencies of the order of five cycles per second, or higher, and consequently introduces an eddy current loss, a disadvantage in a great many measurements. The greatest difficulty has always been to devise an instrument which would accurately record the total change in magnetic flux in the specimen. The ideal instrument would be a fluxmeter with no restoring force and no friction. Fluxmeters are on the market in which the restoring force is negligible only over short periods of time or in which there is no restoring force but where the friction is appreciable; but if it is required that the magnetic cycle have a period of more than a few seconds, such fluxmeters are out of the question.