1893.] 



Magnetic Viscosity. 



355 



been plotted, and their close agreement proves that the difference 

 found between the static and quick cycle curves is not due to the 

 cause suggested by Mr. Evershed. In each case the battery used had 

 a potential difference of 108 volts, the periodic time of the ballistic 

 needle being 10 seconds. 



It was observed, when taking the hysteresis curve by the method 

 in fig. 2, that the sum of the inductions found by varying the 

 magnetising force from one maximum to an intermediate point, and 

 from that point to the other maximum, did not exactly equal the 

 induction got by varying the magnetising force direct from one 

 maximum to the other. 



To investigate this with the ballistic galvanometer the magnetis- 

 ing force (fig. 3) was taken from one maximum through zero to the 

 point a by one motion of the reversing switch handle, and the 

 galvanometer circuit closed at known intervals of time after such 

 change, the deflection being noted. This deflection does not repre- 

 sent an impulsive electromotive force, nor yet a constant current, 

 but is caused by a current through the galvanometer diminishing in 

 amount somewhat rapidly. It might arise from the comparatively 

 slow rate at which the magnetising current changes, owing to the 

 self-induction of the circuit, or it might arise from a finite time 

 required to develop the induction corresponding to a given magnetis- 

 ing force. The former would be readily calculable if the ring had a 

 definite self-induction ; in our case it is approximately calculable. 



Let R be resistance of primary circuit, E the applied electromotive 

 force, x the current, and I the total induction multiplied by the 

 number of primary turns. 



E = Ux+~ • 

 at 



Now I is known in terms of x for conditions of experiment very 

 approximately, and roughly dljdt has a constant ratio to dxjdt — is 

 equal, say, to Jj{dxjdi) ; hence the well known equation 



„ „ , T dx 

 E = l&x + h ^- , 



E , _b 



X = ^ (1 — 6 



From our curves we see that induction per sq. cm. increases 

 10,000, whilst magnetising force increases 4. Total induction multi- 

 plied by the primary turns, taking the volt as our unit, increases 

 10,800 X 200 X 10~ 8 , whilst the current increases ^ an ampere, i.e., 



L = 4-32 x 10- 2 . 



2 c 2 



