620 



ELECTRICAL MEASUREMENTS 



e.m.f. G is a dead-beat galvanometer, and C is an adjustable 

 condenser. The leads a and b are carried to the points between 

 which the P.D. is to be investigated. By inspection of the 

 diagram it will be seen that once on each wave, and at a definite 

 point, the condenser C is charged to the potential existing be- 

 tween a and 6. As the charge is determined by the breaking of 

 the contact, the blocks may be of sufficient width to eliminate 

 the effect of the jumping of the brushes. Also the resistance at 

 the contact will not be of sufficient magnitude to prevent 

 complete charging of the condenser. 



FIG. 385. Connections for synchronous commutator. 



The function of K\ is to discharge the condenser through the 

 galvanometer after Kz has broken circuit. The instrument 

 would ordinarily experience a constant deflection, but the 

 brushes KI and K* are rigidly connected and mounted on a 

 radial arm, which is geared to the shaft so that it moves very 

 slowly. The effect is to gradually move the contact point over 

 the wave. The deflection of the galvanometer will at any instant 

 be proportional to the P.D. between a and b at the instant of 

 breaking at Kz, or in other words, the deflection follows the 

 wave form. 



The actual arrangement is shown in Fig. 384, where the con- 

 tact device, the synchronous motor, and the direct-current motor 

 used for starting the apparatus will be seen. By use of worm 

 gearing the wheel train necessary for moving the brushes is 

 made very compact; the reduction for the instrument shown is 

 7,200 to 1. 



