LOIV VOLTAGE CATHODE RAY OSCILLOGRAI'H 147 



tive values of plate voltage the current to the plates is practically 

 equal to the current in the electron stream and consists largely of the 

 returning electrons. The small current in the other direction when 

 the plate voltage is negative is a measure of the ionization in the tube. 



The Fluorescent Screen 



The screen is spread on the inner surface of the large end of the 

 tube, using pure water glass for binder. The active material consists 

 of equal parts of calcium tungstate and zinc silicate, both specially 

 prepared for fluorescence. This mixture produces a generally more 

 useful screen than either constituent alone. The pure tungstate 

 gives a deep blue light which is about 30 times as active on the photo- 

 graphic plate as the yellow-green light of the silicate, while the silicate 

 gives a light which is many times brighter visually than that from the 

 tungstate. By mixing the two materials in equal parts a screen is 

 produced which is more than half as bright visually as pure zinc 

 silicate and more than half as active photographically as pure calcium 

 tungstate. 



For mechanical strength the end of the bulb which carries the screen 

 is rounded outwards so that the screen is not a plane surface. This 

 introduces a distortion of the fluorescent pattern which in most in- 

 stances is negligible. If the pattern is recorded by a camera whose 

 lens is D cm. from the end of the tube, then the apparent reduction 

 of the deflection produced by the curvature of the bulb is given in 

 terms of the deflection y approximately by 



. 20 + D 3 



^y ^ -460D- y '^- 



The Function of the Gas 



The part which the gas plays in focusing the beam of electrons 

 is an interesting phenomenon which depends upon the difference in 

 the mobilities of electrons and positive ions. The electrons of the 

 beam are pulled toward the common axis by a radial electric field 

 produced by an excess of positive electricity in the electron stream 

 and an excess of negative electricity in the space outside the beam. 

 This distribution is produced as follows: Some of the electrons of 

 the stream, in passing through the gas, collide with gas molecules 

 and ionize them. Both the colliding electrons and the secondary 

 electrons leave the beam but the heavy positive ions receive very 

 little velocity from the impact and drift out of the beam with only 

 their comparatively low thermal velocity. Positive ions, therefore, 



