A Magnetic Curve Tracer 



By F. E. HAWORTH 



An apparatus for pliotograpliicall>' recording hysteresis loops and initial 

 magnetization curves is described. It employs a rotating drum and a tUix- 

 nieter, the restoring torque of the latter being completely counter-balanced 

 by a photoelectric cell arrangement. With this apparatus curves may be 

 taken so slowly that edfly currents are negligible. The accuracy of the 

 instrument is intrinsically as great as that of a ballistic galvanometer. An 

 anaKsis of sources of error is included. 



FOR accurate determinations of hysteresis loops and initial magneti- 

 zation 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 ' describes one 

 which was used with short, thick specimens in a magnetic yolk. 

 Fleming^ 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 ^ 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 fltixmeter 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 appreci- 

 able; 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. In 

 addition they require that the search coil be of such low resistance 

 that it must ha\-e too few turns for use with long thin specimens, in 

 which the flux is small. These difliculties have been o\ercome in the 

 apparatus described below, in which the principal feature is the use of a 



'J. A. Ewing, "Magnetic Induction in Iron and Other Metals," 3d ed., p. 118. 

 2 J. A. Fleming, Proc. Pliys. Soc. Lon., 27, 316-27 (1915). 

 2 J. B. Johnson, Bell System Tech. Jour., 8, 286-308 (1929). 



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