54 BINGHAM AND JACKSON: VISCOSITY STANDARDS 



of pressure. From the observations the viscosity was calculated 

 by the usual formula: 



V = Cpr-C 



T 



in which rj is the viscosity, p applied pressure, p density of 

 liquid, t time of flow, C and C constants abbreviated from the 

 complete viscosity formula. C was found by direct calculation, 

 C by measuring the time of flow of pure water at 20°C. The 

 viscosity of water at 20°C. was taken as 0.01005. 



The viscosity was measured at a variety of applied pressures. 

 To obtain the true effective pressure, the height of liquid in the 

 manometer was corrected for air buoyancy, column of connecting 

 air, and hydrostatic head of the liquid undergoing measurement. 

 In order to be certain that the drainage of the solution was 

 complete, both the time of flow required to empty and that to 

 fill the bulb were measured. Furthermore, the viscosity was 

 found to be independent of applied pressure. To test the cal- 

 culations and corrections the viscosity of water was measured 

 over a considerable range of pressures. The value was found 

 to be constant. 



In order to avoid the arbitrary scales of commercial viscom- 

 eters and the inconvenient magnitudes of the absolute units, we 

 suggest the use of the "centipoise" as a unit of viscosity. This 

 is the one-hundredth part of the c.g.s. absolute unit. The cen- 

 tipoise is almost exactly the viscosity of water at 20°C. (0.01005) 

 and, hence, is at the same time the specific viscosity of any sub- 

 stance referred to water at very near 20 °C. 



The existing data for the viscosity of water have been reviewed 

 in order to obtain a value as far as possible in accord with our 

 present knowledge. The mean value expressed as fluidity is 

 given by the formula : 



<t> = 2.1482 {{t - 8.435) + V8078.4 + (t - 8.435) 2 } - 120 



in which is the fluidity, and t the centigrade temperature. 



The fluidities of mixtures of ethyl alcohol and water expressed 

 as functions of the temperature, and of the weight and volume 

 percentage of alcohol, will be given in the detailed paper. 



