390 
MR. W. CROOKES ON THE VISCOSITY 
they are only able to move around the vertical axis. The upper socket is lined with 
cork and is pressed down by a spring. The lower socket is a steel cup, and a brass 
point is attached to the bottom of the bulb. This device is adopted because in the 
earlier apparatus the friction of glass against cork generated electricity, which inter¬ 
fered with the free movement of the torsion fibre. The glass fibre being only 
connected with the tube at e, rotating the tube on its axis communicates torsion to 
the fibre and sets the mica plate swinging on the same axis without giving it 
any pendulous movement. The mica plate cl is not fastened direct to the suspending 
fibre, but through the intermediary of a twisted aluminium wire,/’, about 12 inches 
long, with a mirror of platinised glass attached to it. The tube opposite the mirror 
is blown into the form of a thin cylinder of about double its original diameter, and the 
centre of curvature is made to coincide as closely as possible with the fibre, or centre 
of motion of the mirror. There are three reasons why the mirror is kept some 
distance above the mica plate, instead of being close to it:—1. There is much less 
distortion of the reflected index of light when passing through a cylinder than 
through a bulb. 2. The light falling on the mirror from the lamp might diffuse 
sufficiently to produce repulsion of the black surface. 3. The torsion bulb and plate 
can be completely packed in wool or immersed in water, to diminish variation of 
temperature. 
The total weight of mica plate, mirror, and aluminium support is 5'69 grains. 
These are purposely made very light, and to render the effects of viscosity more 
apparent the glass fibre is as fine as it can be drawn consistent with safety. The 
diameter of the fibre is about O'OOl inch. The attachment of so thin a fibre at each 
end of the apparatus requires extreme care, or it will snap off at a joint when torsion 
is applied. The following plan has been found to answer best :—The fibre being 
