248 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 3 8 



of attraction. Thus as soon as the electron breaks through the sur- 

 face, it is attracted back and, barring the presence of other fields and 

 forces, it drops back into the cathode. Electron emission was studied 

 by Richardson who reasoned that it was similar to the phenomenon of 

 evaporation of liquids. Richardson suggested an equation now bear- 

 ing his name for showing the magnitude of current emitted. 



The materials used for cathodes which are satisfactory for electron 

 emission are tungsten, thoriated tungsten, and oxide-coated alloys. 

 Pure tungsten cathodes operate at a high temperature ; they are strong 

 and will withstand positive-ion bombardment better than the other 

 emitters. The thoriated-tungsten cathode was developed by Lang- 

 muir and his coworkers. It consists of tungsten containing a small 

 amount of thorium oxide. At a suitably high temperature the oxide 

 is reduced to thorium which diffuses through the metal and forms a 

 layer of pure thorium on the surface of the cathode. The thorium 

 layer, one atom thick is the source of electron emission. Wehnelt 

 found that certain oxides when placed on a metal base became excel- 

 lent emitters at relatively low temperatures. Strontium and barium 

 oxides have proved to be the best. During operation a thin layer of 

 barium forms on the surface of the oxide and this serves as the source 

 of emission of electrons. 



ACTION OF ELECTRON TUBES 



The value of nearly all electron tubes lies in their property of uni- 

 lateral conductivity (that is, the ability to conduct current in one 

 direction but not in the other). This property was shown in the 

 "Edison effect" and was the basis of action of the Fleming valve. 

 The reason for this property can be readily understood. Consider 

 two electrodes placed in a tube having a high vacuum as shown in 

 figure 4. Let F represent the cathode which is heated so as to be 

 emitting a copious supply of electrons and P is the anode connected 

 through a battery to the cathode. If the battery is connected so that 

 P is positive with respect to the cathode, as in a, the negatively charged 

 electrons will be attracted toward P and part of them will land on 

 P, thus constituting an electric current through the vacuum. How- 

 ever, if the battery be connected so that P is negative with respect 

 to F, as in part b, it will repel the electrons and no current will pass. 

 Obviously, if an alternating source of potential be substituted for the 

 battery, electrons will pass to P for those loops of voltage when P is 

 positive but will not do so when P is negative. Thus this two- 

 electrode device becomes a rectifier of alternating current. The 

 factors which control the flow of electrons between the cathode and 

 the anode or plate P are not entirely evident on the surface. Thus 

 the number of electrons which pass to P in a tube having a high 



