158 



PROCEEDINGS OF THE AMERICAN ACADEMY. 



have for any value of / a value which in the case of the statical curve 

 belonged to a current weaker by any given constant or otherwise deter- 

 mined amount. The curve FP must, however, have the same form 

 for a continuously growing current and for one which suddenly begins 

 to increase from the value OE. 



As a matter of fact, experiment seems to show that if the core of an 

 electromagnet is made of varnished wire so fine that eddy currents are 

 practically shut out, the upper portion of a f/" curve with a single inter- 

 mediate step is exactly like the corresponding portion of the V curve. 

 Figure 52 represents a set of current curves obtained from a number 



SECONDS. 



Figure 52. 



Current curves for a coil with fine wire core. The second part of a two-stage 

 current is exactly the same as if the current were allowed to grow at once to its 

 final value. 



of toroidal coils (with very fine wire cores) connected up in series ; the 

 current came from a storage battery of ten cells. When the circuit had 

 its normal resistance, the final value of the current was represented by 

 OA ; it was possible, however, to close the circuit with such an extra 

 amount of resistance that the final value of the current should be repre- 

 seutable on the same scale as before, by the line OK. The extra resist- 

 ance could then be suddenly shunted out of the circuit by closing a 

 switch at any time after the lower current had practically attained its 

 maximum strength. When the core had been previously demagnetized, 

 a diagram of this kind had the form OHDXU ; but if the circuit had 

 from first to last its normal resistance, the current curve had a shape 

 accurately represented — when the starting point was shifted to the proper 



