254 ME. CLERK MAXWELL ON THE VISCOSITY 



In order to determine how much of the total retardation of the motion is due to the 

 viscosity of the wire, the moving disks were placed in contact with each other, and fixed 

 disks were placed at a measured distance above and below them. The weight and 

 moment of inertia of the system remained as before, but the part of the retardation of 

 the motion due to the viscosity of the air was less, as there were only two surfaces 

 exposed to the action of the air instead of six. Supposing the effect of the viscosity 

 of the wire to remain as before, the difference of retardation ic that due to the action 

 of the four additional strata of air, and is independent of the valu° of the viscosity of 

 the wire. 



In the experiments which were used in determining the viscosity of air, five different 

 arrangements were adopted. 



Arrangement 1. Three disks in contact, fixed disks at 1 inch above and below. 



„ 2. „ „ „ 0-5 inch. 



„ 3. Three disks, each between two fixed disks at distance 0G83. 



n ^' 5> »> " » >» U"4Z0. 



„ 5. „ „ „ „ „ 0*18475. 



By comparing the results of these different arrangements, the coefficient of viscosity 

 was obtained, and the theory at the same time subjected to a rigorous test. 



Definition of the Coefficient of Viscosity. 



The final result of each set of experiments was to determine the value of the coefficient 

 of viscosity of the gas in the apparatus. This coefficient may be best defined by con- 

 sidering a stratum of air between two parallel horizontal planes of indefinite extent, at 

 a distance a from one another. Suppose the upper plane to be set in motion in a hori- 

 zontal direction with a velocity of v feet per second, and to continue in motion till the 

 air in the different parts of the stratum has taken up its final velocity, then the velocity 

 of the air will increase uniformly as we pass from the lower plane to the upper. If the 

 air in contact Avith the planes has the same velocity as the planes themselves, then the 



V 



velocity will increase - feet per second for eveiy foot we ascend. 



The friction between any two contiguous strata of air will then be equal to that 

 between either surface and the air in contact with it. Suppose that this friction is equal 

 to a tangential force/" on every square foot, then 



. V 



where p is the coefficient of viscosity, v the velocity of the upper plane, and a the 

 distance between them. 



If the experiment could be made with the two infinite planes as described, we should 

 find jU/ at once, for /a 



