276 



Prof. J. A. Ewing. 



[June 20, 



to the increment of force as at the beginning of the process of 

 magnetisation. The creep np of magnetism was again observed 

 during a minute : then another small step up of the current was 

 made, and so on. The compensation remained nearly correct for 

 a number of steps, but as the process was continued up the curve of 

 magnetisation, it became apparent that the immediate effect was 

 increasing, in other words that there was under-compensation, and 

 that the compensating coil would have to be moved a little forward if 

 an exact balance was to be maintained. The results of this experi- 

 ment are given below (Table II), and are exhibited in fig. 4. The 

 magnetising current was increased from one to another of the succes- 

 sive values shown in the table at intervals of 1 minute in each case, 

 by moderately quick movements of the sliding block in the liquid 

 rheostat. The changes of magnetic force were therefore not quite 

 sudden ; each of them took perhaps a quarter of a second to 

 complete. 







Table II. 







Magnetising current. 



Step. Total. 

 30 30 



Immediate 

 magnetic 

 effect of 

 step. 

 63 



Additional increase 

 of magnetism 

 in 



1 minute. 



36 



Total 

 magnetism 



(after 

 1 minute). 

 99 



13 



43 



27 



23 



149 



12 



55 



26 



22 



197 



12 



67 



25 



35 



257 



23 

 17 



90 

 107 



(49 + ) 

 36 



(75-) 

 53 



381 

 470 



5 



112 



10 



22 



502 



10 



122 



21 



33 



556 



The step of 23 was too large to have its immediate effect balanced 

 by the compensating coil in the position in which the coil was set. 

 The magnetic effect of such a large step is conjecturally shown by 

 the broken line marked (?) in fig. 4. It will be noticed that the 

 points reached after 1 minute at each step lie well on a continuous 

 curve, which is shown by a dotted line in the figure. 



In Table II and fig. 4 one scale division of magnetising current is 

 equivalent to 0*00362 c.g.s. units of magnetising force, and one scale 

 division of the magnetometer is equivalent to 0'0177 c.g.s. units of 

 |j. The immediate value of d^/d^ is about 10, as before, and this 

 applies approximately throughout the range of magnetism dealt 

 with here, with a slight increase towards the upper end of the 

 range. 



Higher up in the curve of magnetisation, however, the immediate 

 effect of a small quick increment of magnetic force is greater, though 

 then (owing to the greater steepness of the curve $ and p) it bears 



