296 FLUIDITY AND PLASTICITY

The value of the constant C is most conveniently obtained by
filling the instrument with freshly distilled, dust-free water and
determining the time of flow for each limb, at 20°C.

„ _ 0.01005* + C'p ,».

C - —^2 W

for water at 20°. This constant may also be obtained by direct
measurement

C = 384.8r4/FZ (4)

where r is the radius of the capillary in centimeters. If the
acceleration of gravitation of the locality is not 980, the value
of C must be increased 0.1 per cent for each unit in excess.

Since the bulbs C and K may differ in level, it is evident that
the pressure, p, used in calculating the viscosity is not necessarily
equal to the pressure, pi, delivered by the compressed air at the
top of the viscometer. If the bulb K is higher than the bulb C
by a distance h, then it is evident that the pressure during the left
limb determination is decreased by an amount hip and the pres-

'; sure during the right limb determination is increased by the same

! amount. Hence,

' Pi - hlP = * + ' ?'p/tl (left limb)

and therefore

+ h,P = " +%p/tt (right limb)

* -JL _

1

_ _

2CP \«! t 2C \tS if) 2

or if the two determinations are carried out with water at 20°
using the same pressure

, 0.005034 /I 1 \ C' I 1 1

where t, is the time of flow from right to left and £2 is the time of
flow from left to right. Log. 0.005034 = 7.70191 — 10.
The value of C used in calculating the hydrostatic head is an
approximate value obtained from Eq. (3) by employing the pres-
sure, pi, uncorrected for hydrostatic head, which is legitimate
since the hydrostatic head is at the most only a small correction
term.
J. W. Temple has worked out a simpler method for calculating