.J02 ROUGH WAYS MADE SMOOTH. 



iron always has some traces of magnetism in it, especially if 

 it has once been magnetised. This weak current being 

 caused to traverse the coil surrounding the soft iron in- 

 creases its magnetism, so that somewhat stronger currents 

 are produced in the revolving coil. These carried round the 

 soft iron still further increase its magnetism, and so still 

 further strengthen the current In this way coil and magnet 

 act and react on each other, until from the small effects due 

 to the initial slight magnetism of the iron, both coil and the 

 magnet become, so to speak, saturated. Machines con- 

 structed on this principle are called dynamo-electric ma- 

 chines, because the generation of electricity depends on the 

 dynamical force employed in rapidly rotating the coils. 



We need not consider here the various forms which 

 magneto-electric and dynamo-electric machines have 

 received. It is sufficient that the reader should recognise 

 how we obtain an electric current of great intensity in one 

 case from mechanical action and permanent magnetism, 1 and 

 in the other from mechanical action and the mere residue 

 of magnetism always present in iron. 



In the cases here considered it is in reality the sudden 

 presentation of the coil (twice at each rotation) before the 

 positive and negative poles of the magnet, which induces 

 a momentary but intense current of electricity. The rotation 

 being exceedingly rapid, these currents succeed each other 

 with sufficient rapidity to be appreciably continuous. A 

 similar principle is involved in the use of what is called the 

 inductive coil, except that in this case the sudden beginning 

 and ceasing of a current in one coil (and not magnetic 

 action) induces a momentary but strong current : matters 

 are so arranged that the current induced by the starting of 

 the inducing current, immediately causes this to cease; 

 while the current induced by the cessation of the inducing 

 current immediately causes this current to begin again : so 

 that by a self-acting process we have a constant series of in- 



1 So called, though in reality the best magnets gradually lose force. 



