624 



ELECTRICAL MEASUREMENTS 



in effect shifted slightly with respect to the wave form, and the 

 charge given to the condenser varies accordingly. The result 

 is the same as that attained in the previous apparatus by shifting 

 the brushes. 



The recording drum is geared to make one complete turn 

 while the motor makes 1,500 revolutions, so that three waves 

 are recorded per revolution of the drum. The pen, if attached 

 directly to the index of the galvanometer, would move on so 

 short a radius that the diagram would be much distorted. This 

 distortion is reduced in the manner indicated in Fig. 388. 



The arm, db, 18 cm. long, is attached to the movable system; 

 by means of a fork it turns the arm, ec, 36 cm. long and pivoted 

 at e. This arm carries the pen at c. The arc ac is much nearer 

 a straight line than is ab and its length is proportional to 0. 



The record obtained is shown in Fig. 389. The fact that it 

 is not on rectangular coordinates is a disadvantage, especially if 

 it is desired to analyze the waves. 



Vm. 



FIG. 390. Connections for integrating method for determining wave form. 



Integrating Methods for Determining Wave Form. 3 One 



arrangement for determining wave form by an integrating 

 method rather than by "instantaneous" contacts is shown dia- 

 grammatically in Fig. 390. 



The current, ii, whose wave form is to be determined passes 

 through the primary of an air-core transformer, in the secondary 

 of which is a low-range voltmeter, or a galvanometer, and the 



