CHAPTER IV. 

 VIRTUAL VALUE OF AN ALTERNATING CURRENT. 



Curve-tracing by Contact Maker. The easiest method of 

 determining experimentally the wave form of an alternating 

 current or electromotive force is to fix a rotating contact 

 maker to the shaft of the generator. This contact maker 

 acts as a switch, which is only closed for an instant once 

 during each revolution of the shaft. If the terminals of the 

 alternator are connected to an electrostatic voltmeter 

 through this contact switch, the voltmeter will indicate the 

 voltage of the machine at the instant when the circuit is 

 closed. By varying the position of the contact maker the 

 voltage of the alternator can be observed for a series of 

 positions of its rotating part, and the variation of voltage 

 during a complete cycle may be traced. 



One of the simplest forms of such a contact maker is a 

 disc of ebonite, with a metal strip let into the edge at one 

 point. Two insulated brushes, each formed of a strip of 

 spring steel, are mounted side by side so as to press on the 

 edge of the disc. Once in each revolution they are connected 

 together by the rotating strip. By mounting the brushes on 

 a movable arm which can be clamped to a divided sector, 

 the point of contact can be varied by any desired angle. 

 A convenient form of this arrangement is shown in Fig. 39. 



Another device which is very easy to apply is to provide 

 a rotating pin on the shaft or coupling of the alternator. 

 The pin comes into contact with a light spring once during each 



FIG. 39. ROTATING CONTACT MAKER. 



