282 



Prof. J. A. Ewing. 



[June 20, 



might perhaps be anticipated from what we knew about static 

 hysteresis, that the immediate effect (d^Jdf) of a step-down is 

 decidedly less than the immediate effect of a step-up. When the 

 compensating coil had been adjusted to balance the first effect of a 

 step-up, it was found to give over- compensation for a step-down. 



Another process has been examined, namely, the alternation of a 

 step-up with step-down, many times repeated. After the magnetis- 

 ing current had been raised to a certain value, it was periodically 

 altered through a definite narrow range by alternately putting in and 

 pulling out the short-circuit plug of a small resistance coil in the 

 main circuit, or by making and breaking a feeble circuit in a second 

 solenoid wound over the first. It was only when this process had 

 been repeated many times that the magnetic effects of the small 

 changes of |§ became approximately cyclic ; the early cycles were 

 associated with a progressive rise in the intensity of magnetism. But 

 when a nearly cyclic state was reached, the compensating coil could 

 be adjusted to balance the immediate effects of + or — <5|p, and 

 the same adjustment of course served to balance either. 



Tested in this way the gradient d^/d^j (for the immediate effect 

 of <5|| after many small + and — steps) has of course a lower value 

 than the gradient which is found when |p is first raised to p + 

 The latter, as we have seen, is greater when the magnetisation is 

 moderately strong than when there is little or none. The former is 

 nearly constant throughout a wide range of |j ; its value is approxi- 

 mately the same as at the initial part of the magnetisation curve — 

 namely 10 — until the region of saturation is approached, when it 

 becomes distinctly less.* 



The periodic changes of magnetism which are brought about by 

 successive small increments and decrements of p exhibit a lagging 

 and creeping up and down precisely similar to that which has been 

 illustrated in fig. 1. That figure may serve to show in a general way 

 the relation of the change of Jf to the change of |f, when at any place 

 in the curve a very small increment £p has been applied and removed 

 often enough to establish a cyclic regime. I have not made any full 

 examination of the variation which under these conditions the 

 gradient d^ld^ suffers when the magnetism on which the small cycle 

 if superposed is gradually pushed up towards saturation, nor of the 

 proportion which the subsequent creeping up or down bears to that 

 part of the change of g which occurs immediately on the application 

 or removal of #g. The creeping which follows each repeated applica- 

 tion and removal of £|$ is certainly much reduced when the iron 

 approaches saturation ; but the immediate effect is also reduced, and 

 so far as may be judged by rather rough determinations, it appears 



* Cf. Lord Rayleigh, loc. cit., on the approximate constancy of tlie static gradient 



