TABLE 150. 



VISCOSITY. 



The coefficient of viscosity is the tangential force per unit area of one face of a plate of the 

 fluid which is required to keep up unit distortion between the faces. Viscosity is thus measured 

 in terms of the temporary rigidity which it gives to the fluid. Solids may be included in this 

 definition when only that part of the rigidity which is due to varying distortion is considered. 

 One of the most satisfactory methods of measuring the viscosity of fluids is by the observation 

 of the rate of flow of the fluid through a capillary tube, the length of which is great in compari- 

 son with its diameter. Poiseuille * gave the following formula for calculating the viscosity coef- 



ir/lf^s 

 ficient in this case : /*= .,, , , where h is the pressure height, r the radius of the tube, s the 



density of the fluid, v the quantity flowing per unit time, and / the length of the capillary part of 

 the tube. The liquid is supposed to flow from an upper to a lower reservoir joined by the tube, 

 hence h and / are different. The product hs is the pressure under which the flow takes place. 

 Hagenbach t pointed out that this formula is in error if the velocity of flow is sensible, and sug- 

 gested a correction which was used in the calculation of his results. The amount to be sub- 



z> 2 

 tracted from h, according to Hagenbach, is -= , where g is the acceleration due to gravity. 



V--S 

 Gartenmeister \ points out an error in this to which his attention had been called by Finkener, 



and states that the quantity to be subtracted from h should be simply ; and this formula is 



S 



used in the reduction of his observations. Gartenmeister's formula is the most accurate, but all 

 of them nearly agree if the tube be long enough to make the rate of flow very small. None of the 

 formulae take into account irregularities in the distortion of the fluid near the ends of the tube, 

 but this is probably negligible in all cases here quoted from, although it probably renders the 

 results obtained by the " viscosimeter " commonly used for testing oils useless for our purpose. 

 The term " specific viscosity " is sometimes used in the headings of the tables ; it means the 

 ratio of the viscosity of the fluid under consideration to the viscosity of water at a specified 

 temperature. 



TABLE 150. Specific Viscosity of Water at different Temperatures relative to Water at C. 



*"Comptes rendus,'' vol. 15, 1842. " Mdm. Serv. Etr." 1846. 



t " Pogg. Ann." vol. log, 1860. 



t " Zeits. fiir Phys. Chim.' ; vol. 6, 1890. 



The value 0.0178 is taken from a paper by Crookes (Phil. Trans. R. S. L. 1886), where the coefficient is given as 

 |t = o.oi7793i/', where P ' = i +.0336793 T-\- .0002209936 7' 2 , where T is the temperature of the water in degrees 

 Centigrade. The numbers in the table were calculated not from the formula but from the numbers in the column 

 headed " mean value." 

 SMITHSONIAN TABLES. 



136 



