hersey: viscosimeters 529 



200 atm. The lard oil was later carried to 500 atm., and its 

 viscosity found to increase much more rapidly than at the lower 

 pressures. These results point to the advisability of now map- 

 ping out the n, p, t surfaces of all lubricants in a systematic 

 manner. In doing so, it is possible that both the above method 

 and the free discharge method suggested below will be found 

 simpler than the immersed capillary tube under differential pres- 

 sure, used by Faust in some recent experiments not embracing 

 lubricants. 



The extension of Poiseuille's law to high pressures. Various 

 equations relating to lubrication might well be generalized to 

 include the effect of pressure. Thus Poiseuille's law may be 

 rewritten 



Q=l~r*C (8) 



o Ho 



in which Q is the volume discharged in unit time, by steady 

 isothermal stream line flow, through a tube of radius r under a 

 pressure gradient G; fi being the viscosity at the outlet pressure, 

 and C a dimensionless coefficient depending on the viscosity- 

 pressure curve of the liquid. Thus 



C= Mo FJP- (9) 



P-Pj?»f(p) 



P being the outlet, and P the inlet values of the pressure p, 

 and ju = f{p) being some empirical formula for the viscosity. 

 By postulating a particular form for /, we could, in theory, eval- 

 uate the coefficients therein (such as a of equation (7) ), by 

 observing the discharge from a high pressure reservoir through 

 a long water-jacketed tube into the free air. 



To determine how nearly isothermal such flow could be made, 

 note that, when K is constant, the final temperature rise of any 

 incompressible viscous fluid, during steady (unaccelerated) adia- 

 batic flow, is rigorously given by the formula 



Ap 

 JK 



in which Ap is the total pressure drop, P -- P , J the mechanical 



At = -5* (10) 



JK 



