86 CURRENT VALUE. 



Alternator armature. 



K Rotating contact maker. 



V Electrostatic voltmeter for reading instantaneous 



value of voltage. 

 C Condenser. 



Fj Voltmeter for reading virtual value of voltage of 

 alternator. 



Instructions. Connect the alternator terminals to an 

 electrostatic voltmeter in series with the rotating contact 

 maker, so that the voltmeter is only momentarily connected 

 to the alternator by the contact maker once during each 

 revolution. It will usually be found necessary to connect a 

 condenser in parallel with the electrostatic voltmeter in 

 order to get reliable readings. Connect a second voltmeter, 

 which need not be electrostatic, to the terminals of the 

 alternator. Insert a regulating resistance in the alternator 

 field circuit. 



Throughout the experiment keep the alternator voltage 

 as read on voltmeter F t constant, by regulation of the field 

 when necessary. 



For a series of positions of the contact maker take readings 

 of the voltmeter F. 



Plot the results as a curve, measuring displacement of 

 contact horizontally and voltage vertically. 



It is probable that difficulty will be experienced in reading 

 the lower voltages on account of the uneven scale of the 

 electrostatic voltmeter. Thus a voltmeter reading up to 80 

 volts cannot be used for voltages much below 20, and a 

 voltmeter reading to 160 will probably have a scale not 

 extending below 80. For taking the lower readings, a small 

 battery may be inserted in the position indicated by B in 

 the lower part of Fig. 40. The voltage of this battery is then 

 in series with the voltage to be measured, and should be 

 chosen of a suitable value to bring even the lowest reading on 

 to the scale. The true value of the voltage to be measured 

 is obtained by subtracting the battery voltage from the 

 voltage recorded on the voltmeter. This auxiliary battery 

 has not to supply any appreciable current, and may conse- 

 quently be formed of very small cells. These should give 

 a fairly constant voltage, and must be well insulated. A 

 battery of small secondary cells answers the purpose well, 

 or a set of small dry cells, which are inexpensive. 



