74 



to sway in a horizontal plane. This reflected beam is received by a 

 totally reflecting prism (or by a mirror), which is kept oscillating about 

 a horizontal axis by means of a synchronous motor ( 68) fitted with a 

 suitable cam-plate, which engages the roller mounted on the tail-piece 

 of the prism. The shape of the cam-plate is such that while the 

 prism is receiving the reflected beam its angular velocity is constant, 

 so that the displacement of the reflected beam in a vertical plane is 

 directly proportional to time. When the prism reaches its maximum 

 displacement, a sector mounted on the motor axle cuts off the light, and 

 during this time the prism executes a quick-return motion, to resume 

 its constant angular velocity in a forward direction as the sector passes 

 out of the path of the beam. 



On leaving the prism or mirror, the light is rendered convergent 

 by a second cylindrical lens, and forms a tiny speck of light on the 

 ground-glass screen which is provided to receive it. This speck is 

 subject to two displacements at right angles to each other, one being 

 due to the vibrations of the oscillograph galvanometer, and the other 

 to the steady forward displacement of the prism (during the return 

 motion, the light is cut off as already explained). Thus the curve 

 traced out on the screen shows the wave-shape at a glance. If it is 

 desired to obtain a permanent record of the wave-shape, a photographic 

 dark slide is substituted for the screen, and an exposure made in the 

 ordinary way, by means of a shutter placed behind the first cylin- 

 drical lens. 



Fig. 56 shows the form of cam-plate used in the Blondel oscillo- 

 graph. The axle of the synchronous motor carries a gun-metal disc, 

 and screwed to this are the two portions of the hardened steel cam- 

 plate. It is important to use hardened steel for both the cam-plate 

 and the roller at the end of the prism tail-piece in order to prevent a 

 distortion of the curve due to wear of these parts. 



The controlling field for the vibrating portion of the oscillograph 

 galvanometer is provided by means of a powerful permanent magnet, 

 shown in Fig. 57. This magnet is fitted with laminated tapering 

 pole-pieces and core, which are separated from each other by two air- 

 gaps, in which are placed the vibrating portions of the instrument. A 

 single gap is quite sufficient if only one wave at a time is required. 

 But frequently it is desirable to obtain simultaneously the records 

 of two waves (such as a p.d. and a current wave), so as to determine 

 their phase relation. Hence most oscillographs are made double. 



In each gap is placed an extremely thin and narrow vertical band 

 or strip of soft-iron, with its width along the lines of force a position 

 which the band would naturally take up if free to rotate about a 

 vertical axis. The tension of the band may be adjusted to the desired 

 amount, and so its natural rate of torsional vibration about its axis 

 controlled. This rate of vibration may correspond to a frequency as 



