for determining the viscosity of very viscous liquids 25 



§ 2. In my experiments I used Dr Searle's viscometer, as 

 supplied by Messrs W. G. Pye and Co., Cambridge*. I determined 

 the viscosity of treacle, as Dr Searle refers to a determination of 77 

 for that liquid. I found 26 = 3-74 cm., 2a = 5-01 cm., and 

 D = 1-95 cm. Since g = 982 cm. sec. -2 at Trondhjem, the con- 

 stant C has the value 



C = 3-070 ± 0-035. 



From the data given by Dr Searle, I find for the constant of the 

 instrument used by him, Cg = 3-153. 



In my instrument the rate of shearing for radius r is given by 



^_ _27r 15-80 

 '^ dr~ T ' r^ ' 



§ 3. To examine how MT depends upon M, when I is kept 

 constant, six series of observations were taken with six values of I 

 varying from 10-0 to 2-15 cm., and in each series M was made to 

 vary from 5 to 205 grammes. 



Since the viscosity of highly viscous substances diminishes very 

 rapidly as the temperature increases, as was shown by Reigerf and 

 by Glaser J for values of rj of the magnitudes 4-8 x 10^ to 67-2 x 10®, 

 and by Ladenburg§ for '>7 = 1-3 x 10^, great care must be taken 

 to keep the temperature constant. The apparatus was, therefore, 

 placed in a thermostat with electric temperature regulation, and 

 a very constant temperature of 19-8° C. was /naintained. The 

 apparatus was left in the thermostat for 24 hours before the 

 measurements were begun, and, during the short time a rotation 

 trial was in progress, only the outer wooden door of the thermostat 

 was opened, since one could see into the thermostat through the 

 inner glass door. The final measurements were all carried out in 

 the course of a day; the observations were made at intervals of 

 about 10 minutes, so that the unavoidable disturbances of tempera- 

 ture, due to the manipulations, might have time to disappear. 



In other respects the measurements were carried out in ac- 

 cordance with Dr Searle's II instructions. The revolutions were 

 timed by aid of a stop-watch and the times were taken for different 

 numbers of revolutions with odd numbers up to 9, as well as the 

 average time for one revolution. As no decrease in the time of a 

 single revolution could be noticed as the rotation continued, the 

 divergences from the mean lying within the limits of the errors 



* Catalogue of Scientific Apparatus manufactured by W. G. Pye and Co., 

 List No. 120, p. 39, 1914. 



t R. Reiger, loc. cit., p. 998. 



X H. Glaser, Ann. d. Phys., 22, p. 719, 1907. 



§ R. Ladenburg, Ann. d. Phys., 22, p. 309, 1907. 



II G. F. C. Searle, loc cit., p. 603. 



