DETERMINATION OF WAVE FORM 



645 



To obtain a uniform sensitivity and a sharply defined spot 

 where the cathode rays impinge upon the screen, it is essential 

 that a constant exciting voltage be used, from 10,000 to 20,000 

 volts being required, unless the cathode is heated. 



The cathode rays proceed perpendicularly outward from the 

 electrode, K } and fall on the target, T, where most of them are 

 stopped. A small pencil of rays, however, passes through the 

 opening and falls on the screen, S, producing a bluish fluorescent 

 spot. Just in front of the target are placed two deflecting coils 



Fl(; 4 o7 -Showing proportionality of deflection and current with a Braun 



tube. 



with their common axis transverse to that of the tube. The 

 magnetic field set up by these coils, when traversed by a current, 

 deflects the cathode stream and Fig. 407 illustrates the relation 

 between the deflection of the fluorescent spot and the current 

 It is seen that the deflection is proportional to the current 

 that it reverses when the current is reversed. ,*{ 



When an alternating current is passed through the deflectmg 

 coils, the spot stretches out into what appears to be a 

 5 nt band If this band is viewed in a revolving mirror the 



