140 



PROCEEDINGS OF THE AMERICAN ACADEMY. 



same asymptote as A. According to this diagram the current attains 

 75 per cent of its own final value more quickly when r is 40 than when 

 r is 20, but B crosses A at the point .v and the current seems to reach 

 practically its full strength sooner in the latter case. The curve C first 

 crosses the curve A and then B. It would be easy to show from a 

 series of oscillograph records for similar cases that the characteristics 

 of the theoretical curves correspond in general to fact. 



TENTHS OF SECONDS. 



Figure 33. 



Forms of current curves for Q deduced from theoretical considerations. The 

 coil has 1394 turns and contains a storage battery of voltage 26. C is everywhere 

 convex upward : A and B have two points of inflexion. 



If with the core of the magnet Q initially neutral a steady current 

 of given strength be established in the coil of 1394 turns, by use of a 

 storage battery of voltage E, the integrand will be for every value of 

 the current inversely proportional to E (since B/r is given), and the 

 building-up time will be inversely proportional to the applied electro- 

 motive force, as it would be if the inductance were fixed. For a given 

 exciting coil, the general shape of the curve for a given current is 

 independent of the applied voltage. Curves A, C, and I) of Figure 34 

 are the current curves computed for E = 26, 52, 104, and r = 20, 40, and 

 80 : the maximum value of the current is the same in every case. G 

 and E are the current curves computed for E = 26, r — 80, and for 

 E= 104, r = 320. 



As has been explained already, it is difficult to obtain an accurate 

 hysteresis diagram for a very large core by the ordinary ballistic 

 methods with such galvanometers as are usually to be found in the 



