THE CATHODE KAY OSCILLOGRAPH 9 



In 1905 Wehnelt ^^ suggested the use of a hot, Hme coated filament, 

 which he had found a couple of years earlier to be a strong emitter 

 of electrons and which was the basis of the present day oxide coated 

 filaments in vacuum tubes. Wehnelt made up such a tube (Fig. 13) 

 which he could operate on the 220 volt power circuit. This was prob- 

 ably the first practical application of the oxide coated filament. A 

 number of experimental tubes were made up with hot filaments in the 



Fig. 13— Wehnelt, 1905. 



years following, but almost twenty years were to elapse before a really 

 successful tube with hot cathode and low voltage was developed. This 

 was the Western Electric 224 tube,^^ to be discussed in greater detail 

 presently. 



Another tube of this type, by von Ardenne and Hartel,^^ is illus- 

 trated in Fig. 14. 



After Braun and Wehnelt, the most notable change in structure 

 was made by A. Dufour ^^ in France in 1914. Up to that time per- 

 manent records of the pattern on the fluorescent screen were made by 

 means of a camera. This usually meant that the pattern for a rapid 

 phenomenon had to be repeated many times before the photographic 

 plate was exposed enough. Dufour omitted the fluorescent screen 

 and instead placed the photographic plate inside the tube so that the 

 cathode ray could play directly on it. When the cathode rays strike 

 the photographic emulsion directly a record can be traced in a much 

 shorter time than when the intermediary light from a fluorescent 

 screen is focused by a lens on the photographic plate. Placing the 

 photographic plates internally of course involved a number of com- 

 plications, such as mechanism for moving the plates inside the evacu- 

 ated tube, means for inserting and taking out the plates, and pumps 

 for producing and maintaining the vacuum. The old glass structure 



10 Wehnelt, A., Phys. Zeit., 6, p. 732, 1905. 



11 Johnson, J. B., //. Am. Opt. Soc. & R. S. I., 6, p. 701, 1922. 



12 Hartel, H. von, Zeit. f. Hochfr. Techn., 34, p. 227, 1929. 

 IS Dufour, A., Compte Rend., 158, p. 1339, 1914. 



