APPENDIX C

329

iust.rum.ent has considerable range, for wires of differing diameters
caix be used for widely differing viscosities. The readings are
instantaneous and the instrument is compact and easily manipu-
lated. The most troublesome feature of this type of instrument
is the lack of constancy in the supporting wire. It is neces-
sary to use these wires with considerable care and to calibrate
frequently. Since the corrections of the instrument are not
fully understood, the calibrating fluid should have nearly the
same viscosity as the viscosity to be measured (cf. Herschel
(1020)).
For liquids of high viscosity, the falling sphere method is used
industrially. If the containing vessel does not have a diameter
a/t least 10 times that of the ball, a correction must be applied
Slxeppard (1917). The method is admirably adapted for abso-
lute measurements, but usually workers have felt dependent
upon calibrating liquids, but since there is a dearth of calibrating
fliiids of high viscosity liquids are often used in which the velocity
of fall is too great for the strict application of Stokes' law and a
correction has to be made. Reproducible liquids of high viscosity
which liave been accurately determined should be available for
tlie industrial requirements.