COEFFICIENT OF FRICTION OF AIR. 251 



133. Coefficient of Friction of Air. When air flows 

 through a long pipe, it has, like water, a resistance of fric- 

 tion to overcome, due to the surface of the pipe ; and this 

 resistance, which is found to consume by far the greater 

 part of the work expended, can be measured by the height 

 of a column of air, which is determined by the expression, 



in which, as in the case of water (Art. 103), / denotes the 

 length, d the diameter of the pipe, v the velocity of the air, 

 and / the coefficient of resistance of friction, to be deter- 

 mined by experiment. The work expended in friction gen- 

 erates heat, the most of which must be developed in the air 

 and given back to it Some heat may be transmitted 

 through the sides of the pipe to surrounding materials, but, 

 in all the experiments that have thus far been made, the 

 amount so conducted away appears to be very small ; .and if 

 no heat is transmitted, the air in the pipe must remain sen- 

 sibly at the same temperature during expansion ; that is, 

 the heat generated by friction exactly neutralizes the cool- 

 ing due to the work done. 



A discussion by Prof. Tin win * of the experiments by Messrs. Culley 

 and Sabine on the rate of transmission of light carriers through pneu- 

 matic tubes, in which there is a steady flow of air not sensibly affected 

 by any resistances other than the surface friction, furnished the value 

 / = 0.038. The pipes were of lead, slightly moist, 2 inches (0.187 

 ft.) in diameter, and in lengths of 2000 to nearly 6000 feet. 



Girard's experiments upon the motion of air in pipes gave a mean 

 coefficient of resistance, / = 0.0256 ; those of D'Aubuisson gave as a 

 mean, / = 0.0238 ; while those of Buff gave the mean value of / == 

 0.0375. 



According to the experiments of Weisbach, it is only when veloci- 

 ties are about 80 feet that the coefficient of resistance can be put = 

 0.024, and it diminishes as the velocity of the air in the pipe increases. 



* See Ency. Brit, Vol. XU., p. 491. 



