482 Mr. W. Sutherland on Thermal 



and dies away indefinitely with increasing exhaustion of the 

 bulb. 



It will be seen that the deflecting force depends on the 

 dimensions of the apparatus in a somewhat complicated 

 manner, but that the most important principle is that (as 

 regards the denominator) it increases with diminishing dis- 

 tance D between vane and glass wall, except at pressures so 

 low that the first two terms are negligible ; and as diminish- 

 ing D means in general an increasing value of b/s, we see 

 that in general at all pressures the efficiency of the radiometer 

 is increased by bringing the edge of the vane nearer to the 

 glass wall. Other things being equal the deflecting force is 

 proportional to the total sectional area E -f S of the bulb. 



As regards the effect of the nature of the gas on radiometer 

 motion the equation shows that at pressures low enough for 

 neglecting the first two terms of the denominator all gases 

 give the same deflecting force, a theoretically interesting 

 result, but not of much practical importance : the practically 

 important matter is to determine how the different gases com- 

 pare, each at its maximum effectiveness ; now when the 

 deflecting force is a maximum P2 + Pi= ^Vo( v 2 +^i) 2 /^-^ r OJ or 

 D = 2A, where A. is the mean path at (p 2 +pi)/2, and the 

 deflecting force becomes proportional to irjo( v 2 + v i) z /^^ v <h so 

 that the most effective gas is that for which r) (v 2 + Vi) 2 /v is 

 largest, that is to say, for which rj /m^ is largest ; compare for 

 instance hydrogen and oxygen, 77 for H 2 is '44 of that for 2 , 

 while rm is 1/4, and thus at the pressure of maximum efficiency 

 H 2 is 1*76 times as efficient as 2 , and at higher pressures the 

 advantage of H 2 increases till its efficiency is 1*76 2 that of O2. 

 From Rayleigh's measurement of the viscosity of helium as 

 •96 of that of air (Proc. Hoy. Soc. Jan. 1896) while hydrogen's 

 is about *5, and with 4 as the molecular mass of helium as 

 against 2 for hydrogen, it would appear that helium ought to 

 be nearly 2* or 1*4 times as efficient in a radiometer as 

 hydrogen. 



The equation (25) contains the laws of the dependence of 

 radiometer motion on the temperatures of the faces of the 

 vanes, although as these temperatures have never been 

 measured experimentally, we cannot verify them as they stand ; 

 but to a certain extent we can bring them within the range 

 of experimental verification in the following manner. When 

 the black face of a vane is suddenly irradiated the tempera- 

 ture of the black face suddenly rises, while that of the clear 

 face is unaltered, and the fall of temperature is confined 

 for the first moment or two to the thickness of the layer of 

 lampblack ; the first deflexion of the vane takes place in 



