CURRENT VALUE. 



85 



revolution, and so completes the circuit. The spring in this 

 case also must be insulated, and provided with means for 

 moving it round a graduated dial. 



A fine jet of conducting fluid has been employed instead 

 of a spring, in order to insure more uniform contact. This is 

 less easy to apply. In either of the latter arrangements 

 contact is usually made to the rotating pin through the 

 frame and shaft of the machine. In some cases this would 

 be objectionable, and the pin must then be insulated, and 

 connected to a slip ring. 



The readings are generally taken upon an electrostatic 

 voltmeter, since the readings of such an instrument are 

 independent of the duration of the time of application of 

 the voltage. It is usually necessary to put a condenser in 

 parallel with the voltmeter, in order to insure steady readings, 

 since the leakage which takes place is often sufficient to dis- 

 charge the voltmeter between successive contacts. By 

 adding a condenser, a practically uniform voltage is main- 

 tained at the terminals of the voltmeter, in spite of a small 

 leakage. In Fig. 39 it will be noticed that the spring brushes 

 are put one in advance of the other, so that the contact is 

 made only momentarily, and not during the whole time 

 necessary for the contact to pass under the brushes. 



Another method of taking the readings is to replace the 

 electrostatic voltmeter by a condenser, which is then dis- 

 charged by a switch through a ballistic galvanometer, when 

 a reading is to be made. By this method a momentary 

 throw, instead of a steady reading, is obtained. 



EXPERIMENT XV. DETERMINATION OF WAVE FORM OF 

 AN ALTERNATOR. 



DIAGRAM OF CONNECTIONS. 



