36 ELEMENTS OF ELECTRICAL ENGINEERING. 



intervals between contacts of b 1 and s without perceptible de- 

 crease of voltage across the condenser terminals. 



The sector // is shifted step by step around the divided circle, 

 and the circle readings and voltmeter readings are taken. A 

 complete cycle of electromotive force values is obtained by shift- 

 ing the sector pp over the angular distance between two adjacent 

 north poles (or south poles) of the field magnet, and in plotting 

 the electromotive force curve the circle readings are laid off as 

 abscissas and the voltmeter readings as ordinates. 



To determine a current curve by means of the contact maker, 

 the device shown in Fig. 33 is mounted on the armature shaft of 



Fig. 34. 



the alternator which supplies the current, the current is made to 

 flow through a non-inductive resistance R, the curve of the 

 electromotive force across the non-inductive resistance is deter- 

 mined, and the ordinates of this electromotive force curve are 

 divided by R to give the points on the current curve. 



(b) By the oscillograph. In order that a galvanometer needle 

 may indicate the successive instantaneous values of a rapidly 

 changing current, such as an alternating current, the galvanometer 

 needle must be extremely light and the period of free vibration 

 of the needle must be extremely short. The oscillograph is a 

 galvanometer constructed to meet these requirements. 



The Duddell oscillograph consists of two very fine wires, WW t 



