466 BELL SYSTEM TECHNICAL JOURNAL 



the work function is not a characteristic of a given substance but 

 rather of a given surface of a given substance. Strictly speaking, one 

 should not talk about the work function of tungsten but rather of the 

 work function of a particular surface of tungsten. This is true even 

 if the surface is clean tungsten. 



The Values of the Work Function for Clean Surfaces 



The experimental determination of the thermionic work function 

 or the heat function for clean metal surfaces has been the subject 

 of many investigations. In the case of a number of elements, the 

 determinations by different investigators are not in accord. This is 

 due, in most cases, to adsorbed layers of foreign material caused by 

 either poor vacuum conditions or impurities in the metal which have 

 not been eliminated by a proper heat treatment. Although these 

 measurements have been summarized and discussed in other reviews, 

 it seems advisable that the summary be brought up to date and the 

 most probable values selected from the existing data. Since the 

 photoelectric work function is equal to the thermionic work function,^ 

 the determination by photoelectric methods should also be included. 



A summary of the data is shown in Table IV. The values of the 

 photoelectric work function and the thermionic heat function are 

 expressed in volts. The reference for each value is indicated by the 

 superscript. As discussed in an earlier section, the heat function is 

 the slope of a Richardson line. The photoelectric work functions are 

 mostly calculated from the long wave-length limit except in the case 

 of recent determinations which are made by an analysis of the data 

 by Fowler's ^"^ method. The photoelectric values listed in the table 

 were selected as representative of values for the best outgassing of each 

 element. For a listing of all determinations see Hughes and Du- 

 Bridge's book. In most cases, the heat function and the thermionic 

 work function differ only by small amounts so that for practical pur- 

 poses we can compare the photoelectric work function with the heat 

 function. The most probable values of the heat functions tabulated 

 have been chosen from the several determinations. 



Recently several attempts have been made to find an empirical 

 r.elation between the work function and the atomic properties of the 

 elements. Such a correlation, if applicable to all of the metallic 

 elements, would be of value in predicting values of the work function 

 for the cases in which the existing data are inadequate or no data are 

 available. The work of Rother and Bomke ''■- gives the best correlation 

 thus far obtained. In their article they have given a summary of the 

 early attempts at a correlation and therefore we will not consider 

 them here. 



