548 



Dr. John Ilopkinson 



[April 26, 



These phenomena of the times at which changes of induction 

 occur in the cores of electromagnets have a close analogy to the 

 retardation of signals upon cables. Many of you know if a battery 

 is connected to a submarine telegraph cable, and if its connection 

 is suddenly reversed, that the current throughout that cable is not 

 reversed at once, but that it takes an appreciable time before the 

 effect is perceptible at the far end of the cable. This retardation of 

 the signal, as it is called, rapidly increases with the length of the 

 cable. I will first of all show you two curves of arrival, as they are 

 called, of the current in a cable (Fig. 10). In these curves the 

 ordinates represent the currents, and the abscissas represent the 

 times. You will observe that there are two curves shown ; one is 





Fig. 11. 



for a length of cable double the other, the longer cable giving the 

 slower times. Now all that the curves in this form are capable of 

 showing you is that with the longer cable the events are more 

 retarded. In order to make them really comparable with the 

 magnetic experiments, we must show a curve in which the abscissae 

 represent, not the currents, but the rate of change of current. We 

 have such a curve here (Fig. 11). In this case the abscissa represent 

 the rate at which the current is changed, and the ordinates represent 

 the time. This curve has a remarkable similarity to the magnetic 

 curve with the lowest current. The similarity of the curves, as 

 shown in Fig. 12, is quite sufficient to suggest an analogy between 

 the phenomena which actually exist. I can show you the same 

 thing in another way. There are in the library one of Lord 



