DETERMINATION OF WAVE FORM 



635 



(period about K,soo second), one for the current, the other for 

 the potential. They have the necessary adjustments for properly 

 locating the spots of light on the screen. No damping arrange- 

 ment is employed. The rotating multisided mirror is driven l>v 

 gearing from a simple synchronous motor which is easily started 

 by giving the mirror a twist just as the switch is closed. The 

 arc, which is enclosed, can be readily adjusted as to position and 

 requires little attention. Small carbons are used and a very 

 intense and well-defined spot of light is obtained. 



Galvar.omflter 



FIG. 399. Demonstration oscillograph. 



With the screen at a distance of 12 feet, the amplitude of the 

 wave is about 2 feet, while the length of the wave is 2 1 A feet. 

 Both the shunt and the multiplier are adjustable so that the am- 

 plitude of the wave may be varied. 



Naturally, as the period of an oscillograph galvanometer is 

 very short, the sensitivity is low. In the laboratory form of 

 instrument a deflection on the scale of from 1 to 3 mm. usually 

 corresponds to about 0.01 ampere. 



The movable mirror is very small so an intense source of light 



l uired. A direct-current arc is commonly employed. 

 I .earn of light, after being reflected from the oscillograph mirror, 



