CURRENT VALUE. 



87 



An additional advantage gained by the addition of the 

 battery is that it enables a distinction to be made between 

 positive and negative values of the voltage, since the battery 

 will be in series with the voltage when acting in one direction 

 and in opposition to it when it changes sign and the volt 

 curve crosses the zero line. Without such a device it is 

 often difficult to judge the exact point of reversal, since the 

 voltmeter readings are always in the same direction, 

 irrespective of reversal of the applied voltage. A key 

 should be arranged for throwing the battery directly on to 

 F, so that its voltage may be read at short intervals, and 

 subtracted from the total readings. 



The same method of tracing curves may be applied to 

 read the instantaneous values of the current in an alternating 

 circuit, and so to obtain the current wave form. In this case 

 the terminals of a non-inductive resistance in the circuit are 

 connected periodically to the voltmeter by means of the 

 contact maker. In a non-inductive resistance the voltage 

 is always proportional to the current, and consequently the 

 voltmeter readings when divided by the constant resistance 

 give the values of the current. 



By means of a throw-over switch the simultaneous 

 values of current and voltage in a circuit may be obtained by 

 connecting the contact maker and electrostatic voltmeter 

 alternately to the terminals of the alternator, so as to read 

 the voltage of the circuit, and to the terminals of a non- 

 inductive resistance, so as to read the voltages proportional 

 to the current. 



This is the principle of the following experiment. 



EXPERIMENT XVI. DETERMINATION OF SIMULTANEOUS 

 CURVES OF CURRENT AND VOLTAGE IN A CIRCUIT. 

 DIAGRAM OF CONNECTIONS. 



e R 



~^-r- AA/VW 



vs 



P' 



FIG. 41. 



