616 



ELECTRICAL MEASUREMENTS 



is superposed on the battery voltage. If a reflecting instrument 

 is used, the calibration curve is very closely a parabola, and as 

 the upper part of it is practically a straight line the deflection 

 from the false zero is sensibly proportional to the voltage between 

 a and b at the instant of contact. 



Fig. 380 shows the arrangement when a ballistic galvanometer 

 is employed. 



In this arrangement Si is a double-pole, double-throw switch, 

 by which the apparatus may be connected to the alternating- 

 current circuit, or to the direct- current circuit for purposes of 

 calibration. Ci is a variable condenser which is charged by 

 throwing S 2 to the left, and discharged through the ballistic 

 galvanometer, BG, when the switch is thrown to the right. 



The deflection, which is proportional to the instantaneous 

 voltage between a and 6, may be controlled by varying the 

 capacity. 



Use of Potentiometer Principle. These methods may be 

 improved upon if a potentiometer arrangement is adopted, as 

 shown in Fig. 381. 



FIG. 381. Contact method for wave form, using potentiometer 



principle. 



Referring to the figure, DE is a slide wire, or its equivalent, 

 which is supplied with direct current from B. The voltage from 

 to D or E, is slightly larger than the maximum occurring be- 

 tween a and b. A direct-current voltmeter is connected between 

 and the slider S; G is a detector, which may be a telephone 

 or a moving-coil galvanometer; a Kelvin instrument is not 

 suitable. 



After setting the contact brush, the position of the slider S 

 is varied until the detector G stands at zero. The instantaneous 



