PAPERS OxN PHYSICS 305 



A WEHNELT CATHODE FOR THE EMISSION OF A 



SMALL, COMPACT AND PERMANENT 



BEAM OF ELECTRONS 



Prof. C. J. Lapp, University of Illinois 



A hot line Wehnelt cathode has been constructed by- 

 means of which a very small and compact permanent beam 

 of electrons may be secured through the application of 

 barium resinate and strontium hydroxide to a platinum 

 strip. Heretofore when an experiment was performed re- 

 quiring a small beam, tw^o methods were available, one in 

 which a tiny speck of either calcium chloride or Bank of 

 England sealing wax was used and the other in which a 

 small beam is obtained from a larger one by means of a plat- 

 inum diaphragm. The disadvantage of the former is that 

 the beam is not permanent, lasting at times but a few min- 

 utes ; while the latter can not be used to advantage in strong 

 magnetic fields since the beam is deflected to one side and 

 fails to pass through the opening in the diaphragm. 



A strip of platinum 0.5 mm. wide was cleaned with nitric 

 acid and ammonium hydroxide. A tiny drop of strontium 

 hydroxide was placed on the strip, after which it was dried 

 by gently heating the strip by means of an electric current. 

 After two applications the platinum strip was heated red in 

 order to harden the deposit. A small and almost micro- 

 scopic piece of barium resinate was then placed centrally 

 on the spot and the whole carefully heated so as to evapo- 

 rate the resin and have barium oxide. After two or three 

 coats of barium oxide the strip was glowed to cherry red 

 for several minutes in order to drive off all organic material. 

 A coating of approximately 0.1 mm. in diameter was thus 

 obtained which gave an intense and compact permanent 

 beam of electrons without the use of a diaphragm. A beam 

 of this nature has been long sought after in this laboratory 

 and is now available for a number of uses. 



Laboratory of Physics 

 University of Illinois 

 April, 1921. 



