THE ALTERNATOR. 



37 



Fig. 34, stretched very close together between the poles NS of 

 a strong magnet. A very light mirror M is attached to the 

 wires, and the current to be measured flows down one wire, 

 across the connection c and up the other wire, so that one wire 

 is pushed forwards and the other wire is pushed backwards by 

 the magnet, thus deflecting the mirror. A beam of bright light 

 falls on the mirror and is reflected to a moving photographic 

 plate upon which is left a permanent trace of the movements of 

 the mirror. The alternator A, of which the electromotive force 



Fig. 35. 



curve is to be determined, is connected through a non-inductive 

 resistance R to the oscillograph as shown in Fig. 34. 



To determine a current curve, the oscillograph is connected to 

 the terminals of a non-inductive resistance through which the 

 current flows. 



An example of electromotive force and current curves taken 

 simultaneously by two oscillographs is shown in Fig. 35-* The 

 curve e shows the electromotive force across the terminals of an 

 alternating arc, and the curve i shows the current flowing through 

 the arc. The fine waves in the electromotive force curve are due 

 to the free oscillations of the needle. 



* Reproduced from a paper On Arc Lamps by Andr6 Blondel, Transactions of 

 the International Electrical Congress, St. Louis, 1904, Vol. II, pages 731-767. 



