7l6 INDUSTRIAL APPLICATIONS. 



2nd. The previous determination of the function /(a) will give 



*(!)-* [/()-/()}. 



These two methods have the inconvenience of giving the electro- 

 motive force which corresponds to zero current in the ring, and 

 leave aside the effect due to the magnetisation of the ring. 



3rd. The machine being in motion and producing the current I, 

 the ring is pressed by a sort of forked contact, consisting of two 

 branches kept at a distance of two strips by an insulating plate, 

 and which communicate with a galvanometer of high resistance g. 

 The difference of potential e a of two successive strips is equal to 

 the excess of the corresponding electromotive force e a over the 



product of the resistance r of the coil by the current - ; the 

 current / in the galvanometer gives then 



I I 



t a = ty.-- r = tg, or e ai = ig+-r. 



Starting from one of the brushes, we shall successively determine 



*0 ^2J3) ^> ---- *(2m-l)/3 



and we shall get 



In the present case, each of these values of e is the algebraical 

 sum of the electromotive forces corresponding to the two flows of 

 force of the field and of the ring. 



4th. The electromotive force of a machine is sometimes es- 

 timated by the current I which it produces in a circuit of total 

 resistance R + x, by the help of the equation E = I(R + #), and 

 from it is deduced the characteristic function 



but this method of working is not quite accurate, for the resistance 

 of the induced ring should be considered as containing a fictive 

 term, almost proportional to the velocity of rotation, and this cal- 

 culation cannot be neglected in calculating the electromotive force. 



