58 ELECTRIC LIGHTING. 



of the axis of the bobbin, on which press two springs con- 

 nected with the circuit. 



When it is wished to use cylindrical cores of iron there is. 

 an advantage, in respect to induction, to form them of iron 

 tubes, slit longitudinally, and to put two or three of them 

 one within the other. In the large machines of the Alliance 

 Company this plan has been made use of. 



FIG. 



As to the more or less advantageous effects of quicker or 

 slower alternations of magnetization and demagnetization, 

 the question is complicated on account of the magnetic 

 inertia of iron. According to the proportionality of the 

 tension of the currents to the derivative of the function of the 

 time, one would suppose that these alternations ought to be 

 as rapid as possible ; but the tardiness with which iron is 

 magnetized and demagnetized much complicates the effect 

 produced, and it is observed that whilst slow interruptions of 



