when an Electron escapes from Surface of a Hot Body. 219 



This condition prevailed in the Richardson and Cooke 

 experiments. The question immediately arises, Why did 

 equation (9) not hold for their values? A seemingly 

 reasonable explanation may be offered, as follows : — It 

 has been shown that the presence of active gases always 

 tends to increase </>. Also that inert gases do not affect </>. 

 The low value they found for the cooling effect for osmium 

 would indicate that the gases active with osmium had been 

 mostly consumed, so that the cooling effect measured must 

 have been that peculiar to the material under ideal con- 

 ditions. If the true value of </> for the two metals was the 

 same, then the <f) actually existing for the cold metal must 

 have been larger than that for the osmium under the 

 conditions of the experiment. If this were true, then the 

 order of the metals in the voltaic series was reversed, and 

 equation (9) should have been written 



0, = <£ 2 -V c , 



which would make the heating effect greater than the 

 cooling effect by an amount comparable with the order of 

 magnitude of the contact potential difference. The observed 

 difference was 1*5 volts. This is of the required order of 

 magnitude. Since there was no counterbalancing film at 

 the osmium surface, this potential difference is greater than 

 would ordinarily exist. 



5. A Method for Measuring Temperatures of Filaments. 



The thermionic current is an exponential function of the 

 temperature. Owing to the rapid variation of the current 

 w r ith the temperature, it ought to be possible to measure high 

 temperatures very accurately by observing the thermionic 

 currents, since these may be determined very accurately *. 

 The method has not been employed, partly because the 

 emission properties of hot surfaces have not been found to 

 be constant. There has been a further difficulty in the fact 

 that independent temperature determinations have been 

 necessary to determine the current-temperature curve for 

 any wire. The work of Langmuir, and more recently that 

 of Smith, have shown that under ideal conditions the 

 emission properties of hot bodies are constant. The diffi- 

 culty in regard to calibration may be obviated if b in the 

 current- temperature equation is determined by measuring (p. 



* Qt. 0, W. Richardson, Phys. Rev, vol. xxvii. p. 183 (1908). 



