DIFFERENT MODES OF GENERATING CURRENTS. $5 



4. That the direct currents have a shorter duration than 

 the inverse currents. 



5. That it follows from this last property that the inverse 

 and direct currents, though formed by equal quantities of 

 electricity, are able to act differently ; for the direct currents 

 having a shorter duration than the inverse currents, have a 

 greater tension, and may therefore act on a circuit of greater 

 resistance. 



6. That the duration of the direct current is independent 

 of the resistance of the induced circuit, whilst that of the 

 inverse current increases with that resistance and with the 

 number of spires in the coil; whence it follows that in ordinary 

 cases the electro-motive force of the direct current must be 

 greater than that of the inverse current, and that the 

 ratio of these forces increases with the length of the induced 

 wire. 



Induced currents may, like battery currents, be changed 

 into tension currents or into quantity currents, not only ac- 

 cording to the mode in which the induced coils are con- 

 nected, but also according to the insulation of the wire, its 

 thickness, its length, and the composition of the magnetic 

 core causing the induction. With a wire very rine, very long 

 and insulated with all the precautions adopted for the elec- 

 tricity of the glass-plate machines, sparks of more than a 

 metre in length have been obtained ; and with a magneto- 

 electric machine having a wire of large diameter a current 

 may be obtained possessing sufficient quantity to produce the 

 effects of the voltaic battery. 



The laws of induced currents with regard to the effects 

 they produce through the external circuit are the same as 

 those of voltaic currents, but it must be admitted that in this 

 case the resistance of the generator is represented by a quantity 

 much greater than that which can be deduced from its direct 

 measurement if voltaic currents were employed. Thus the 

 calorific work supplied by an induction machine is expressed 

 by Joule's formula, in which the value of the resistance of 



