Convective Cooling of Wires. 333 



refer respectively to the absolute temperatures of the body 

 and its surroundings. 



(b) Convection constants of gases. — Below are given data 

 to express the variation of k and v with temperature. 



The viscosity (rj) of a gas is given by Sutherland's formula 

 and the density by p = p (273/T), where T is the absolute 

 temperature : from these the kinematical viscosity v ( = 77//0) 

 has been derived. 



The conductivity (k) has been calculated from the viscosity 

 (17) and the specific heat at constant volume (c v ), using the 

 formula k/c v r/= constant, as given by the kinetic theory of 

 oases. Eucken * has shown that the constant has the value 

 2*50 for monatomic gases and T90 for the diatomic gases 

 hydrogen, oxygen, nitrogen, and also for air. The specific 

 heats at high temperature have been obtained from the data 

 given by M. Pier f, who has given formulae which, for the 

 above gases, may be written c„ = A(l + 0-0002T), where A 

 depends on the nature of the gas. Langmuir J adopted 

 this method of calculating conductivity when dealing with 

 heat convection from the point of view of conduction through 

 a film of gas adherent to the hot body. 



Thus, for the gases considered, 



v = v/P, • • ( 6 ) 



k = V90c v , (7) 



where ^=KT*/(1 + C/T), (8) 



c B =A(l + 0-0002T), ' (9) 



p = 2n Po /T, (10) 



and where the constants have the values given in the following 

 Table : — 



Table I. 

 (c.g.s. units.) 





10 6 K*. C* 



A. lOVo- 



Air 



Hydrogen 



Oxygen 



15-0 124 



6-6 1 77 



17-5 127 



1 



0-161 , 1293 

 2-21 0-09 

 0-145 1-43 



* See Tables by Kave and Laby, or Fisher, Phys. Rev. xxiv. p. 385 

 (1907) ; xxix. p. 146 (1909). 



* Eucken, Phys. Zeit. xii. p. 1101 (1911). 



t Pier, Z.f. Elektrochem. xv. p. 536 (1909) ; xvi. p. 899 (1910). 

 X Langmuir, loc. cit. 



