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SCIENCE NEWS 



DR. LANGMUIR ON ELECTRON EMISSION 



Eecent discoveries in elecitroii emission and 

 eurrent-contTol were described in a series of three 

 lecftures by Dr. Irvdng Langmuir at Oarnogie 

 Institute of Technology, Pittsburgh, on Novem- 

 ber 27, 28 and 29. Dr. Langmnir's auditors 

 during the series were scientists, engineers, indus- 

 trial executives and students of Carnegie Tech, 

 and his world-wide reputation as a research phys- 

 icist with the General Electric Laboratories at 

 Schenectady attracted caipaeaty audiences. Synop- 

 ses of Dr. Langmuir 's three leotures follow: 



ELECTKON EMISSION PROM HEATED METALS 



When metals are heajted in high vacuum, elec- 

 tron, or atoms of negative electricity, evaporate 

 firom their surface. If theire is another electrode 

 on ifche evaluated space which is given a positive 

 charge the electrons drift over to this electrode 

 (anode) so that a current flows between the two 

 electrodes. Dushman has recently derived an 

 equation wliich should supersede the well-known 

 Biohardson equation, giving the relation between 

 the electron current and the temperatmre of the 

 cathode. The advantage of this new equation is 

 that there is only one constant which we need to 

 know for each different cathode material, instead 

 of two constants which were necessary for the 

 Biehardson equation. 



The electron emission from a large number of 

 different materials has recently been measured. 

 The thoriated tungsten cathode gives a current oA 

 a temperature of 1,500° absolute, which is about 

 130,000 itimes greater than that from ordinary 

 tungsten. Measurements have also been made of 

 cathode materials that have even much greater 

 emissions. 



In order to get all the current that a cathode 

 is capable of giving, it is necessary to apply to 

 the anode a high enough voltage to overcome what 

 is known as the space charge effect. By putting 

 in gases positive ions are formed in the space 

 between the electrodes, and these neutralize the 

 negative space charge and allow the current from 

 the cathode to pass across the space with much 

 lower anode voltages. In other words, the effect 

 of gases is to increase the current- carrying ca- 

 pacity of the two. Such an effect is used in the 

 TiiTign.r rectifier. Care must be taken what gas 

 is used for the .purpose, for many gases have the 



effect of poisoning the cathode, and cutting down 

 its emission to a small value. 



If very high voltages are used on the anode, so 

 as to produce intense electric fields, it is possible 

 to pull electrons out of the cathode. In fact, it is 

 possible to p;ull electrons even out of cold cath- 

 odes, that is, cathodes at ordinary temperatures. 

 The currents obtained this way from the cathode 

 come from very minute areas, but in these areas 

 the current density amounts to more than one 

 hundred million amperes per square inch. 



ELECTRON EMISSION FROM THORIATED FILAMENTS 



The thotriated tungsten filament is a tungsten 

 filament containing one or two per cent, of tho- 

 rium, usually in the form of oxide. When such a 

 filament is heated, to about 3,500° Centigrade, a 

 little of the thorium oxide is changed into metallic 

 thorium. In the meantime, however, any thorium 

 on the surface of the filament evaporates ofE, 

 leaving only pure tungsten. If the filament tem- 

 perature is then lowered to about 1,800°, the 

 thorium gradually wanders or diffuses through the 

 filament, and when it reaches the surface, if the 

 vacuum is very perfect, remains there and gradu- 

 ally forms a layer of thorium aitoms which never 

 exceeds a single atom in thickness. The thickness 

 of this film is therefore about 1/100,000,000 of 

 am inch, and yet this film increases the electron 

 emission of the filament more than one hundred 

 thousand fold. 



Of course this useful film is very sensitive and 

 needs some protection to keep it in good condi- 

 tion. Very sligiit traces of water vapor or other 

 gases would oxidize this film and destroy it. This 

 can be avoided by putting in the bulb some sub- 

 stance that will combine with the water before 

 this has a chance to attack the thorium film. Such 

 a substance is metalHe magnesium. Furthermore, 

 it is necessary to avoid heating the filament to 

 too high a temperature for otherwise the film 

 might evaporate off. It is thea-efore best to oper- 

 ate such filament within a rather narrow range of 

 temperature close to 1,700° C, where the ratio 

 of evaporation is very small, and where the tem- 

 perature is high enough for the thorium gradually 

 to diffuse to the surface and continually repair 

 any damage done by the effect of slight traces of 

 residual gases. 



The thoriated tungsten filament opens up many 

 new fields of scientific invesitigation. By measur- 



