Mr. T. H. Blakesley on Magnetic Lag. 41 



7rM 



If 2T is the period of alternation, -^- is the maximum 



rate of increase of M. If 



XT 7rM -^ ~ 

 N -yr = DC, 



N may be called the coefficient of magnetic self-induction. 



Under the exigencies of a certain temporary nomenclature 

 CD might be called the Foucault-Ampere turns existing in 

 the core itself. 



Substituting for M in terms of AD, 



~ n N7r 4-ttAD 

 DL = ~T iT~' 



or 



DC , , J^Ut 



I have explained above how tan </> may be obtained from 

 the dynamometer observations. We may therefore employ 

 this formula for the determination of the value and constancy 

 of N, if we can rely upon the values of T obtained by obser- 

 vation at the time, and of p known otherwise. 



The constancy of p obtains so long as magnetization can 

 keep pace with magnetic stress. These conditions are fairly 

 well understood, and AD must not exceed the stress repre- 

 senting the limiting one. Thus it will be well to know AD 



AD = AB sin 6 =«iI 1 Vl- cos 2 0, 



1 V A«jBa 2 ' 



vB« J 



I think', therefore, experiment should move in the following 

 direction. The constancy or the reverse of N under varying 

 speeds should be first determined by experiments with currents 

 so small or coils so few that the magnetic resistance p may 

 be safely assumed constant. For this purpose it would be 

 necessary to employ some speed-indicator. 



Professor Forbes, F.R.S., in his paper upon transformers, 

 already quoted, says that the existence of hysteresis would 

 cause a departure from the harmonic character of the motion, 

 but that the effects are small and negligible. 



If N, however great, remains constant, the harmonic cha- 

 racter of the variation is maintained. But if experience 



