154 Prof. 0. W. Eichardson on the 



these, the uncertainty as to the exact law of absorption 

 of the electrons inside the metal and the approximation 

 involved in taking certain factors outside the integral 

 have already been alluded to. In addition, it is likely that 

 7 will be a little different for the electrons liberated by light 

 of a given frequency when isotropic and when incident 

 normally. It is, however, quite improbable that any of 

 these considerations can introduce erroneous factors large 

 enough seriously to affect the order of magnitude of the final 

 result. 



Proceeding in the way indicated above,, it appears that 

 the value of the complete photoelectric emission current 

 density for platinum at 2000° K. should be 



t=2*l x 10~ n amp. per cm. 2 ..... (7) 



Let us now compare this value with the observed thermionic 

 electron emission for platinum at the same temperature. In 

 considering these questions platinum has been used because 

 it is the only element for which the requisite photoelectric, 

 thermionic, and optical data are available. There is one 

 further point. The value (7) has been obtained on the- 

 assumption that the hot body is in an enclosure surrounded 

 by radiation in equilibrium with it at its own temperature. 

 This condition is not satisfied in the thermionic measurements. 

 Most of the electromagnetic radiation emitted by the hot 

 body never returns to it. The radiation density inside the 

 hot body will thus be something less than that assumed in 

 calculating the value (7). Thus (7) is to be regarded, subject 

 to the limitations as to accuracy already referred to, as an 

 upper limit to the photoelectric contribution to the electron 

 emission from the hot body. 



The following values of the thermionic current densities 

 at 2000° K. from a number of elements, including platinum,, 

 have been found by Langmuir under very good conditions 

 as to freedom from gaseous contamination: — 



Element — > W. Ta. Mo. Pt. C. 



Thermionic current o^-in-3 7v1 a-3 19vin -3 fivin -4 in - 



(amp. per cm. 2 ). ~" > 3xl0 7xl ° 13x10 6xl0 10 * 



These are enormously greater than the value (7) of the 

 complete photoelectric emission calculated for platinum at 

 this temperature. The smallest thermionic currents from 

 platinum ever recorded in the neighbourhood of 2000° EL 



