DETERMINATION OF WAVE FORM 



647 



Another disadvantage is that there is necessarily considerable 

 inductance in the deflecting coils. 



A second method of introducing the time coordinate is to use 

 two sets of deflecting coils with their axes at right angles to each 

 other and perpendicular to the axis of the tube. One set of coils 

 is traversed by the current whose wave form is desired, the other 

 by a current of a simple and known 

 wave form. This auxiliary current 

 must vary synchronously with the un- 

 known current, and it may have a 

 linear wave form, the current strength 

 increasing uniformly with the time, or 

 it may have a sinusoidal form. 



Zenneck 7 uses the linear wave, 

 which he obtains by the synchron- 

 ously rotated slide-wire arrangement 

 sketched in Fig. 409. The effect is 

 to cause the fluorescent spot to 

 progress regularly across the screen 

 with every revolution of the slide 

 wire. At the same time the regular 



deflecting coils cause the spot to be deflected in a perpendicular 

 direction. The result is that the curve appears to stand still on 

 the screen. It can be photographed by an ordinary camera or 

 traced on a suitable transparent screen, the eye being kept in 

 a fixed position. Perfect action of the brushes is, of course, 

 essential. 



ff 



To Oolls 



FIG. 409. Diagram of 

 rotary slide wire for use with 

 Braun tube to introduce the 

 time coordinate. 



FIG. 410. Electrostatic tube for determining wave form. 



Electrostatic Tubes.-The cathode rays may also be deflected 

 electrostatically. To this end two condenser plates are moun 

 so that the cathode stream passes between them, the plates being 

 either inside or outside the tube. The deflection is proport.o 

 to the potential difference between theplates and may be adjust 

 by varying their distance apart. For very high voltages a c 

 denser multiplier must be used. Ryan and Mmton have em- 



