317 
The apparatus consists of a disc of metal supported in its 
axis by an elastic wire, the torsion of which causes the disc 
to oscillate when a motion of rotation is imparted to it. At 
the lower end of this axis is rigidly attached a hollow cylin- 
der, open at both ends, which is immersed in the liquid to 
be tested. The damping effect which the liquids examined 
have upon the motion of the disc furnishes a most sensitive 
measure of their viscosities. The motion being quite slow, 
and its direction being frequently reversed, there is little 
tendency to the formation of currents, and external friction 
is reduced to that of the air and the viscosity of the wire. 
Moreover, small particles of solid matter have little influ- 
ence, as there are no corners and no projecting points against 
which they impinge. , 
The arcs of oscillation of a body suspended inthis way 
diminish in a geometrical series, the ratio of which depends 
upon the relation between the moment of inertia of the 
suspended body, the coefficient of torsion of the wire and 
the resistance to which it is subjected. 
For the purpose of determining the length of the arc of 
oscillation, a circular protractor is attached to the upper 
surface of the disc, and a pointer fixed to the frame of the 
apparatus, directly above but not in contact with the scale. 
The scale is numbered on the inside of the graduation 
from 0 to 360°, and on the outside the numbering is con- 
tinued to 720° so that any arc may be directly read up to 
two complete revolutions. The point of equilibrium is ad- 
justed by turning the support to which the upper end of 
the wire is attached until the zero point is directly beneath 
the stationary index. The reading may be greatly facili- 
tated by a suitable magnifying glass placed directly above 
the index. 
The essential parts of the apparatus should be so propor- 
tioned that the time of oscillation between two successive 
turning points will not be less than twenty seconds, and 
when very close results are essential the time of oscillation 
should be a minute or more, in order that the readings may 
be taken with accuracy. 
The general construction of the apparatus is shown in 
figure 1, in which A represents the suspended disc; B, the 
scale consisting of a card board protractor 8 inches in 
diameter, and divided to half degrees ; C, the stationary 
index; D, a large bulls eye lens; E, the resistance cylinder; 
F., vessel containing the liquid tested; C, the suspending 
wire; H, attachment of the wire: I and K, frame which in- 
closes and supports the apparatus. 
