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



In the experiments made E = 4 and 108 volts and R = 0'8 and 

 21 # 6 ohms, whence 



80 2160 



x = 5(l — e 4-32) and 5(1 — 6 432). 



In either case x does not differ sensibly from its final value when 

 t = •§- second. Hence the self-induction of the circuit can have 

 nothing to do with the residual effects observed. 



These experiments showed that an effect was produced upon the 

 galvanometer needle, appreciable for some seconds, the effect being 

 somewhat more marked with 4 than with 108 volts. But the whole 

 amount was so small as to be less than 1 per cent, of the total change 

 of induction; from which we infer that no material difference exists 

 between curves of induction determined by the ballistic galvano- 

 meter and the inductions caused by magnetising forces operating for 

 many seconds. 



Effect of tapping the Specimen. — Having taken the magnetising 

 force from its maximum through zero to the point a as before, the 

 effect of tapping was marked, especially in the case of soft iron, 

 when a kick corresponding to an acquirement of 633 lines of induc- 

 tion per sq. cm. was observed. 



The following experiments on the hard steel wire ring were 

 carried out with the alternator, the object being to ascertain if a 

 time effect on magnetism exists. The ballistic curve (fig. 3) has 

 been taken as a standard with which to compare the respective 

 hysteresis curves. In each case the maximum magnetising force has 

 been made as nearly as possible to agree with that used when taking 

 the ballistic curve, and the method of test was that employed in the 

 ' Electrician ' paper. For the sake of completeness the diagram, 

 fig. 4, and description are given over again. 



Quoting from that paper, we have : " For determining the points 

 on the closed curve of magnetisation given by rapid reversals of the 

 current in the coil, the ring was connected in series with a non- 

 inductive resistance to the poles of an alternate current generator, 

 or a transformer excited by the generator, thus : — 



Fig. 4. 



E 



