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Prof. J. A. Ewing. 



[June 20, 



of this is that the wire responds with great sluggishness to the 

 additional load, and this sluggishness is greater the longer has been 

 the preceding interval during which the load has been maintained 

 constant. To test whether, in like manner, the prolonged application 

 of a constant magnetising force would produce what may be called 

 magnetic hardening, I have made comparative observations of the 

 time- rate of change of magnetism when a definite small increment of 

 force is applied, the preceding force having been kept constant (a) for 

 a short time and (b) for a long time. The result is to show that the 

 process of magnetic creeping after a small step is much slower when 

 the preceding force has been in action for a long time than when it 

 has been in action for only a short time. 



The following experiment illustrates this well. After raising the 

 magnetising force to between 2 and 3 c.g.s. units, the compensating 

 coil was adjusted to balance the immediate effect of a small increase 

 of force, this increase being brought about by short-circuiting 

 1 ohm (out of many ohms) in the magnetising circuit. When the 

 compensation had been adjusted, the iron was demagnetised by 

 reversals, and the magnetising force was again gradually applied. 

 When it reached the value of 2*54 c.g.s., a pause was made for 

 3 minutes, during which time this magnetising force of 2'54 remained 

 constant. The resistance in the circuit of the magnetising current 

 was then suddenly reduced by 1 ohm, which had the effect of raising 

 the force to 2*60. The compensating coil prevented this step-up of 

 magnetising force from having any instantaneous effect on the 

 magnetometer ; but creeping, of course, began at once, and the time- 

 rate of creeping was observed during 10 minutes. Then the 

 magnetising current was kept constant for 50 minutes more, making 

 1 hour in all, and a second step-up of magnetising force was effected 

 by removing another ohm of resistance : the second step was very 

 nearly equal to the first, and raised the force to 2'66 c.g.s. The 

 time-rate of creeping which followed it was also observed during 

 10 minutes. The results are shown in fig. 7, where the curve A 

 shows the growth of magnetism during 10 minutes when the step 

 had been preceded by a 3-minute interval of constant force, and the 

 curve B shows the growth of magnetism when a sensibly equal step 

 was made, which had been preceded by a 1-hour interval of constant 

 force. The times are in each case reckoned from the instant at which 

 the step was made, and the increment of magnetism is in each case 

 reckoned from the value reached just before the step was made. 

 The immediate effect of each step (balanced by the coil) was equiva- 

 lent to 51 scale divisions of the magnetometer. The creeping-up in 

 10 minutes was equal to no less than 531 scale divisions in the case 

 of curve A, as against 320 in curve B. At the place marked with an 

 asterisk in curve A, it happened that the laboratory door was slammed, 



