54 ELECTRIC LIGHTING. 



which are continuous, are manifested during the whole period 

 of the magnet's movement, and their direction depends upon 

 that of this movement, but it always corresponds with a de- 

 magnetization current, that is to say, with a current in the 

 same direction as that of the magnetic current of the core 

 affected.* It is these currents which act in the Gramme 

 machines. The other reactions which were first discussed 

 have given birth to magneto-electric machines, of which the 

 best-known forms are those of Dujardin, Breton, Duchenne, 

 Wheatstone, Breguet, &c. ; but as these machines have not 

 produced effects sufficiently powerful to form generators of 

 electric light, we shall say no more about them. 



There are yet other sources of induction which have pro- 

 mised to give origin to magneto-electric machines, and of 

 their number is that which has given rise to the peripolar in- 

 duction machine of Le Roux; but these sources are still too 

 feeble to be applied advantageously. 



Laws of Induced Currents., Many experiments made 

 on induced currents have proved : 



i. That the quantity of electricity put in action in a circuit 

 is proportional, other things being equal, to the intensity of 

 the inducing current and to the length of the induced circuit. 



2. That it is independent of the duration of the inducing 

 action, and varies only with the magnitude of the initial cause 

 of induction. 



3. That the tension of the induced current varies on the 

 contrary with the duration of the inducing action, and in- 

 creases with the rapidity of the variation of the inducing 

 cause; a circumstance which goes to show that the tension 

 of an induced current is proportional to the algebraic deriva- 

 tive of the function of the time expressing the law of succession 

 of the values of the intensity in the induced current. 



* For the explanation of these effects see Note A at the end of the book. 



