166 PHENOMENA, ATOMS, AND MOLECULES 



molecules is undoubtedly very complex. The error involved in using this 

 equation must, however, be relatively small. ^^ 



Before substituting (5) in (9) let us insert the numerical values of the 

 constants. In (5) we place M = 2, R = 8.32 X 10'^ ergs/° C. and ex- 

 press p in mm. of mercury and w in grams per sq. cm. per second. We 

 thus obtain 



m^^-^^, (10) 



In (9) we place Cy = 5.26 g. cals. per degree (from 300° to 1500° K.)^^ 

 and d = 0.00706 cm. This gives, when we combine with (10) 



T2-T. = 49.6^^^. (II) 



This equation enables us to estimate the difference between the tem- 

 perature of the wire and that of the gas molecules which strike its surface. 



We may also calculate the temperature of the gas molecules" striking 

 the wire in another way. Between the surface C (Fig. i) and the bulb, 

 the ordinary laws of heat conduction must apply at pressures below that 

 at which convection occurs. Thus we may place, according to (2) and (7) 



If we know the effective diameter of the bulb and the mean free path 

 of the gas molecules, we can calculate from this equation the value of 

 (^>a ~ <Pi), and since (p is a known function of the temperature, this in 

 turn will enable us to determine T„. 



Let us now calculate T^ from our experiments with hydrogen by means 

 of Equations 11 and 12. For this purpose we will choose the data ob- 

 tained at 1500° K., for at this temperature there is no appreciable dis- 

 sociation and the temperature measurements are more accurate than at 

 lower temperatures. These data, as taken from Table I and II of Part I, 

 are given below in Table V in the second column. The figures represent 

 the watts per centimeter carried by the hydrogen from a wire at 1500° K. 



^^ With large differences of temperature over distances comparable with the free 

 path, another effect, which we may call the radiometer effect, enters to render Equation 

 5 inaccurate. The rapidly moving molecules leaving the wire tend to drive back the 

 slower incoming molecules and thus to decrease the rate at which the molecules strike 

 the wire. Although this effect would be very important if we were dealing with large 

 flat surfaces, calculation shows that in our present experiments, where only fine wires 

 are employed, this error is always less than 5% and usually much less than this. It 

 has, therefore, been neglected. 



*^ The letter K is used to denote temperatures on the absolute scale (Kelvin). 



