for determining the viscosity of very viscous liquids 33 



formula must be multiplied, according to Hagenbach*, by a cor- 

 recting factor slightly less than unity. As the thermostat could 

 only accommodate tubes shorter than 45 cm., Hagenbach's correc- 

 tion was calculated, but was found to be negligible. Ladenburgf 

 points out that both Hagenbach's and Couette's corrections to 

 Poiseuille's formula can be entirely ignored for liquids such that t] 

 is of the magnitude 1-3 x 10^. 



The discharge vessel consisted of a wide glass cylinder; through 

 the bottom of this was bored a hole through which the discharge 

 tube was connected with the interior of the cylinder. The whole 

 apparatus was placed in the thermostat and the same temperature, 

 19-8° C, was" maintained as was used in the earlier experiments, 

 AVhen a tube whose internal radius was about 0-26 cm. was used, 

 the liquid did not issue in a continuous jet but in drops. The 

 values obtained for -q are given in Table 5. The mean value is 

 r] = 271-1. The value obtained by Searle's method, viz. 274-7, 

 differs from that obtained by Poiseuille's method by 1-3 per cent.; 

 the agreement may be regarded as good. 



Table 5. 

 Values of 7] by Poiseuille^s method. 



§ 8. The influence of the base of the rotating cylinder can be 

 eliminated, without determining k, by using the relation J 



77 = C . ^ -, = Cy, 



n ~ h 



provided that the points corresponding to M^T^ and M^T^ He to 

 the right of the parabolic boundary line in Diagram 1. If we put 

 Zj = 10-0 cm., we obtain the results given in Table 6. 



* F. Kohlrausch, Lehrbuch d. praktischen Physik, pp. 264 — 269, 1914. 



t R. Ladenburg, loc. cit., p. 298. 



i Compare C. Brodman, loc. cit., p. 163. 



VOL. XX. PART L 



