364 SCIENCE PROGRESS 



Theory of Matter " and " Emission of Ions from Hot Bodies ") ; 

 in papers by Langmuir and his co-workers which are summarised 

 and dealt with from the new standpoint in a comprehensive 

 manner in the Trans. Amer. Electrochemical Soc, vol. xxix 

 (191 6), pp. 125-82 ; and in two contributions by Millikan, one 

 to vol. vii of the Physical Review (1916), pp. 18-32, and the 

 other to last September's issue of the same journal. 



What Richardson succeeded in establishing against con- 

 siderable opposition and scepticism was that the emission of 

 electrons from a heated metal is an intrinsic property of the 

 metal and is not caused by secondary chemical reactions, nor 

 is it dependent on or influenced by an electrostatic field around 

 the metal. He showed that if a positive field is maintained 

 around a metal body by bringing near to it an electrode charged 

 to such a positive potential that no electrons will return to the 

 metal surface, then the saturation current thus produced across 

 the surface is equal to «T*e~*^^, where a and b are constants. 

 This equation can be explained by reference to kinetic theory, 

 assuming that the conduction electrons in a metal are ordinarily 

 held within the metal by an electric force at the surface, just 

 as the molecules of a liquid are prevented from escaping by 

 the cohesive force which gives rise to surface tension. There 

 will thus be an amount of work to be done on an electron 

 before it can be freed from the influence of the surface and 

 can escape under the small accelerating field to the surrounding 

 anode. Let this work be w ; then kinetic theory shows that 

 the number of electrons which escape per sec. per sq. cm. 

 of the metal surface is N(^T/27rm)*e~"'/*^, where N is the 

 number of conduction electrons in each c.c. of the metal and 

 k is the gas-constant per molecule {i.e. the Boyle-Charles 

 constant R for a given amount of gas divided by the number 

 of molecules in this amount), whose value is about i'2iS x io~^^. 

 The connection between this formula and the previous one 

 is obvious, and gives the interpretation of the constants a and b. 

 Many experiments were carried out by other workers which 

 seemed to show that this thermionic emission was due to 

 chemical action ; but the recent work of Langmuir under such 

 extreme conditions of vacuum that chemical action between 

 the metal and any residual gas was entirely inadequate to 

 explain the effects observed, have succeeded in demonstrating 

 that the emission is an intrinsic property of the metal, and 

 that w has characteristic values for each metal. If e is the 

 electronic charge, wje (or <f> as it is denoted by Richardson) is 

 also characteristic, and is a measure of a sort of potential 

 difference between the inside and outside of the metal ; but, 

 in view of what will be mentioned presently, P.D. is hardly a 

 correct term to apply to it. 



