358 Dr. Hopkinson and Messrs. Wilson and Lydall. [Apr. 20, 



5 per second. Hence also we infer that this effect, as already described 

 in the 4 Electrician,' is a true time effect, not arising in any way 

 from the ballistic galvanometer. 



In fig. 6 the hysteresis curves for a frequency of 72 are compared 

 with the ballistic curve. The curves are marked 72L and 72H 

 respectively, the potentials at the terminals of the alternator in the 

 two cases being approximately 36 and 430 volts. The corresponding 

 values for B and H are given in Tables III, IV, which have been 

 obtained from the curves in figs. 9 and 10 respectively. 



The difference between the two curves in fig. 6 was at first 

 puzzling, but a little consideration satisfied us that it arises from the 

 same time effect. The curve 72L was determined three times, with 

 the same result. The numerals refer to thirtieths of a half-period. 

 From 26 to 28*8 of the L curve the magnetising force increases from 

 31'8 to 45 - 6, whilst from 21 to 26 of the H curve it increases from 

 30" 6 to 44, the rate of change being about double as great in the 

 former case as in the latter, and it is the L curve which deviates 

 most from the ballistic curve. In like manner, in the neighbourhood 

 of zero induction, the induction in the H curve is changing twice as 

 fast as the induction of the L curve, and it is here the H curve which 

 differs most. How these differences of rate of change arise can be 

 seen by inspecting figs. 9 and 10. 



In fig. 7 the hysteresis curves for a frequency of 125 are compared 

 with the ballistic curve. The curves are marked' 125L and 125H 

 respectively, the potentials at the terminals of the alternator being 

 approximately 62 and 750 volts. The corresponding values for B 

 and H are given in Tables V and VI, which have been obtained from 

 the curves in figs. 11 and 12 respectively. 



These curves show the same difference as fig. 6, but Less markedly 

 than in fig. 5. The L curve was determined twice. 



Experiments have been made upon chromium steel, supplied by 

 Mr. Hadfield, having the following composition: — 0*71 per cent, 

 carbon, 9'18 per cent, chromium, when annealed, and when hardened 

 by raising to low yellow and plunging into cold water. The results 

 show that the same time effect exists in this case, although it was not 

 so marked as in the case of the hard steel. 



We draw the following conclusions from these experiments : — 

 (1.) As Professor Ewing has already observed, after sudden change 

 of magnetising force, the induction does not at once attain to its full 

 value, but there is a slight increase going on for some seconds. (2.) 

 The small difference between the ballistic curve of magnetisation with 

 complete cycles and the curve determined with a considerable fre- 

 quency, which has already been observed, is a true time effect, the 

 difference being greater between a frequency of 72 per second and 

 5 per second than between 5 per second and the ballistic curve. 



