THE ALTERNATOR. 



35 



10. The determination of electromotive force and current curves. 



(a) By the contact maker. The contact maker, whose essen- 

 tial features are shown in Fig. 33, is a device for repeatedly con- 

 necting a large condenser, C, to the terminals of an alternator at 

 a certain instant in an electromotive force cycle, thus keeping the 

 condenser charged up to the voltage e that exists at the given 

 instant in the cycle, in order that the value of e may be meas- 

 ured by a voltmeter V. 



A disk of insulating material is fixed to the armature shaft of 

 the alternator of which the electromotive force curve is to be de- 



Fig. 33. 



termined. A thin metal brush b' rubs on the edge of this disk 

 and makes momentary contact once per revolution with a narrow 

 metal "strip s which is set in the edge of the disk. The brush 

 b 1 is supported on a sector // which slides around on the 

 inner edge of the divided circle cd, and the reading on the divided 

 circle of the mark M indicates the position of the brush b' . 



One terminal of the condenser C is permanently connected 

 to one collecting ring r of the alternator by means of the brush 

 b, and the other terminal of C is connected to the brush b f 

 and thence through the strip s and wire a to the other col- 

 lecting ring r' . 



The voltmeter V must take a very small current in order 

 that the condenser may be able to supply the current during the 



