[ 41 ] 
III. Experiments on Fluid Viscosity. 
By A. Mallock. 
Communicated by Lord Kelvin, P.R.S. 
Received July 26,—Read November 21, 1895. 
These experiments consisted of the measurement of the moment transmitted, by fluid 
viscosity, across the annular space between two concentric cylinders, one of which 
revolves while the other is stationary. 
The fluid used in these experiments was water. Three distinct sets of conditions 
were tried, viz.:— 
(1) Outer cylinder revolving, inner cylinder stationary. 
(2) Inner „ ,, outer ,, ,, 
(3) Repetition of series (l) with an annulus of different width. 
It had been intended to have had a repetition of series (2) with a larger annulus, 
but the motion in the fluid in series (2) was so thoroughly unstable that this was not 
done. 
The object of the experiments was chiefly to examine the limits between which the 
motion of the fluid in the annulus was stable, and the manner in which the stability 
broke down. For obtaining the actual value of the coefficient of viscosity, other 
methods, such as the flow through capillary tubes, would be more suitable. 
Fig. 1 is a section of the cylinders as arranged for the experiment of series (1). 
The inner cylinder A is suspended by a torsion wire, attached by a gymbal ring to 
the top of the stem B. This stem is guided by two rings of balls, held by the gun- 
metal casting C, the rings being adjustable in their own planes by means of four set 
screws at each ring, thus allowing the axes of the suspended and revolving cylinders 
to be made coincident.* The stem B carries a divided circle which is read by the 
telescope T. E is the outer cylinder, carried on the axis F. This cylinder was 
driven at constant speeds by a small electromotor connected with a governor which 
cuts oft the current when, and not before, the desired speed is attained. Surrounding 
* Though this adjustment was made with ease in the main expei’iments, preliminary experiments 
with the axes of the cylinders parallel, but separated by known intervals, had shown that an error in 
centreing the cylinders, if small, produced a difference of the second order only in the moment trans¬ 
mitted by the fluid. 
MDCCCXCVI.—A. G 
10.3.96 
