636 ELECTRICAL MEASUREMENTS 



Fig. 400, passes through a long cylindrical lens which compresses 

 the beam vertically. This focusing improves the definition of 

 the spot of light on the screen and a still further improvement is 

 effected by placing a narrow vertical slit between the arc and the 

 mirror. 



For visual observations the beam of light after passing through 

 the cylindrical lens is received on a multisided mirror which is 

 rotated at the proper speed by a synchronous motor, or else on 

 a mirror which is tilted with a uniform angular velocity by a cam, 

 also driven by a synchronous motor. With the tilting mirror 

 arrangement a shutter actuated by the motor cuts off the light 

 while the cam is returning the mirror to its initial position. 



Screen 



Cylindrical 



Lent A \ . Stationary 



-A Mirror 



Synchronous 

 Mirror 



Total Reflecting 

 ___'J?1^ 



I 

 Arc 



FIG. 400. Showing optical system of oscillograph. 



The revolving mirror furnishes the necessary time coordinates- 

 From it the spot of light is reflected upon a curved translucent 

 screen which is concentric with the axis of the mirror. 



With periodic phenomena the waves appear in a fixed position 

 on the screen, and may be traced on thin paper. 



For photographic work these mirror arrangements are dis- 

 pensed with and the spot of light is focused by the cylindrical 

 lens directly on a photographic surface which is carried either by 

 a falling plate or by a uniformly rotating drum. With the drum 

 a shutter is employed which remains open while the drum makes 

 one revolution. The mechanical features of these v recording 

 devices are described in the catalogues of various instrument 

 makers. 



As it is necessary to decrease the inertia of the moving parts 

 of the oscillograph vibrator as much as possible, the principle 



