362 PHENOMENA, ATOMS, AND MOLECULES 



TABLE IV 



Experimental Test of Constancy of the Ion Current at High 



Filament Temperatures 



\^oltage = 45; £ = 3000 volts cm~'; caesium pressure 

 ° 2.4 X 10-5 baryes; a^M^ = 3.4 X 10>-. 



It is seen that from 1143° to nearly 1400° the current remains remark- 

 ably constant, but rises increasingly rapidly at higher temperatures. This 

 rise has been found to be due to photoelectric emission from the adsorbed 

 caesium film on the cylinder under the influence of the light radiated from 

 the filament. Such photoelectric emission is also observed when light is 

 allowed to fall on the tube. 



Since the light intensity of each wave-length varies in accord with 

 Wein's law, the logarithm of the intensity is a linear function of the 

 reciprocal of the filament temperature, the slope of the line, for natural 

 logarithms, being Co/A, where the radiation constant Co is 1-433 cm deg. 

 Analyzing the data for i^ in Table IV it is found, in fact, that the observed 

 current ip can be resolved into two parts, one having the constant value 

 300.3, and the other, the photoelectric current, being given by ? 



logio (/photo) =9.08-12,270/r. iZS) 



The 3rd column contains values of iphoto calculated by this equation. 

 The 4th column shows that the sum of these two currents, 300.3 -f- iphoto 

 agrees well with the observed value of ip. 



By immersing the bulb in liquid air the ion current became zero, but 

 the photocurrents remained and were found to be 15 per cent lower than 

 those given by Eq. (35), so that the constant 9.08 in this equation needed 

 merely to be changed to 9.01. 



Repeating the experiments of Table IV, with —310 volts on the 

 collector instead of —45, gave currents which, analyzed in the same way, 

 gave a constant ion current having the same value (300.3) as before, but 

 gave photocurrents 2.2 times as great as with the weaker field, the co- 

 efficient of \/T in Eq. (35) being unchanged. By placing the bulb in 

 liquid air these photocurrents were reduced as l^efore by 15 percent. 



