THE VISCOMETER

77

depending upon the direction of the flow. The corrections are
made as for absolute measurements and the viscosity calculated
from formula (22). In obtaining the pressure correction due to
the average resultant hydrostatic pressure in the viscometer C'
can be estimated accurately enough by means of rough measure-
ments. The value of C can be
obtained accurately enough for the
calculation of this correction by
assuming pQ = p. After obtaining
the value of the hydrostatic head x
in this way, the true value of C may
be calculated from an observation
upon the time of flow of any liquid
whose viscosity is accurately known.
In the use of any relative instru-
ment, it is important that two stand-
ards be employed so as to obtain a
check upon the method. For this
purpose a single liquid may be used
at widely different temperatures or
two or more liquids may be used of
widely different viscosities. While
this test is very simple and its
importance is obvious, it does not
appear to have been frequently
employed.

Viscosity Measurements of
Liquids above the Boiling-point.—
If the viscosity of liquids is to be
measured above the ordinary boiling
temperature, one must work at pressures above the atmospheric
pressure. The three-way cocks in Fig. 22 must lead to a low-
pressure reservoir, this pressure being measured by a second
manometer. The rubber connections must of course be replaced
by others capable of withstanding the desired pressure.

Viscosity Measurement of Very Viscous Substances.—Sub-
stances like pitch which are excessively viscous can yet be
measured by the efflux method by the use of very great pressure
(c/. Barus (1893)). On account of the lack of proper drainage,

SecfionV-Yl

FIG. 30.—Plastometer. For use
or with plastic