THE VISCOMETER

75

•where C and C" are constants, which caa be calculated, we obtain

r, •+

C'pfi,

cu

wheace,

In subsequent calculations it is necessary to know the specific
gravity of the liquid whose viscosity is desired, in order to make
the necessary pressure correction and in order to make the kinetic
energy correction, but it is to be noted that if the
instrument has been constructed with that end in
view, these corrections will both be small, and there-
fore the specific gravity need be only approximately
known, which is a great advantage.

Relative Viscosity Measurement. — On account
of the labor involved in obtaining the dimensions
of the viscorneter, many investigators have followed
the example of Pribram and Handl in disregarding
these dimensions, and calibrating the instrument
with same standard liquid. The most important
instrument of this class, is that of Ostwuld, Fig. 28.
It consists essentially of a U-tube with a capillary
in the middle of one lirnb above which is placed a
bulb. A given volume of liquid is placed in the
instrument and the time measured that is required
for the meniscus to pass two marks one above and
one below the bulb under the influence of the hydrostatic
pressure of the liquid only.

If 770 is the viscosity of the standard liquid and T? that of liquid
to be measured, we have from Eq. (22)

FIG. 28.—
The Ostwald
viscometer.

'/O Vy//OHI ^ P(J/ m

and if -r\ is very nearly equal to T?O or if t and fa are very large, this
may be written

1=

170

(23)