52 ELECTRIC LIGHTING. 



of De Meritens, which have furnished such marvellous re> 

 suits. 



Of these various machines, it is evidently those which 

 call a motor into play that are applicable for electric lighting,. 

 for it is in this form that electricity can be obtained under 

 the most economical conditions. These machines will there- 

 fore exclusively occupy our attention ; but before this we 

 think some indications should be given of the laws regulating 

 induced currents, and of the different causes which concur for 

 their production. 



THE DIFFERENT MODES OF GENERATING INDUCED 

 CURRENTS, AND THE LAWS WHICH GOVERN THEM. 



Besides the induction effect which we have already ex- 

 plained, there are many other causes capable of developing 

 induced currents. Every action, the effect of which is to 

 diminish or to increase the power of a magnet already acting 

 on an induction coil may give rise to induced currents which 

 will be direct when there is diminution, and inverse whea 

 there is increase. This increase may result from the action 

 on the magnetic poles of an armature of soft iron, and dimi- 

 nution will result from the withdrawal of that armature. 

 Again, if the pole of a permanent magnet be passed before 

 an iron core surrounded by a coil, a double action will be 

 produced : i, a current which may be termed an electro- 

 dynamic induction current, which will result from the successive 

 passage of the spires of the induced coil before the pole of 

 the inducing magnet, and which will be the more energeti- 

 cally developed the more suitable precautions are taken to 

 avoid the hurtful induction to which the parts of the spires 

 behind those directly affected by the magnet are liable ; 2% 



