GRAPHIC RECORDING INSTRUMENTS 



559 



and the contact is reestablished, the result being that the pen 

 remains practically stationary if the voltage be constant. If the 

 voltage falls, the upper contact and PI act to bring the pen to its 

 proper position. Sparking at the contacts is eliminated by use 

 of the condenser KK and the resistance R. 



18 1 



FIG. 342. Elements of Arconi curve-drawing wattmeter. 



Rectilinear motion of the pen is obtained by making the arms 

 of the linkage, a, b and c of equal length. The upper end of c 

 slides in a vertical slot. 



Registering instruments are made for both direct- and alter- 

 nating-current circuits, to measure current, voltage, power, 





