t 34 ] 



V. On Magnetic Lag, 

 By Thomas H. Blakesley, M. A* 



IN bringing my views on Transformers before the Physical 

 Society it is my desire to emphasize : — 



(1) How the magnetic lag, if it exist, may be measured by 

 employing dynamometers of low resistance. 



(2) That the magnetic lag has a real existence. 



(3) That the magnetic lag necessarily accompanies an 

 absorption of work involved in the reversal of polarity in the 

 iron, and how this may be measured. 



(4) The points in the general argument where scientific 

 facts are wanting, and the direction which investigation should 

 take to meet this want. 



The possibility of the existence of a magnetic lag renders 

 the problem a different one from that of two coils acting and 

 reacting upon themselves by means of mutual and self- 

 induction, whose coefficients, being geometrical, are con- 

 stant. 



For the latter problem I gave in the year 1885 a complete 

 solution, but I pointed out that the completeness of the result 

 rested upon the absence of anything in the nature of hysteresis 

 (a word not then in use) or work done in the field. 



The following year Mr. George Forbes, F.K.S., gave what 

 should have been (but for the very poor reporting of the 

 Society of Arts' Journal) a solution of the " secondary gene- 

 rator " problem, treating it as a case of two coils, assuming 

 that " the magnetism of the core varies as the sum of the cur- 

 rents in the two coils;" and the same gentleman has treated the 

 subject again in a recent paper before the Society of Telegraph- 

 Engineers and Electricians, in which he makes the same 

 assumption, and says, referring to the harmonic functions 

 which he attributes to the electrical and magnetic quantities 

 involved, that the existence of magnetic hysteresis would 

 cause departure from the harmonic character, but that, being 

 insignificant so long as the magnetic induction in the iron is 

 not high, its consideration may be omitted ; statements which 

 seem rather to evade than to overcome the difficulty. 



Mr. Gisbert Kapp, who has done so much good work in the 

 practical development of transformers, also, in my opinion, 

 makes the same assumption, that the state of magnetization in 

 the core coincides with the magnetic stress resulting from 

 compounding the stresses derived from the two coils. 



* Communicated by the Physical Society : read May 12, 1888. 



