298

FLUIDITY AND PLASTICITY

where h is the height of the bulb and PQ is the pressure with all
other corrections made. Fortunately if the height of the bulb
BD, in Fig 29, is not more than one-thirtieth of the whole pressure,
this correction is unnecessary to attain the desired accuracy of
0.1 per cent.

In any case, however, the student should determine by experi-
ment whether a change in manometer pressure is without effect
upon the valve of C.

THE PRESSURE CORRECTIONS OUTSIDE THE VISCOMETER

Let the density of the liquid within the manometer be po at a
temperature T in degrees Centigrade and the height read on the
manometer scale—corrected for scale error if necessary—be hQ;
also let the viscometer bulbs be at a height h' above the middle
point of the manometer. The pressure delivered to the air in
the viscometer becomes

pl = ho — K ± L for a water manometer (8)

Pi = M ± N for a mercury manometer (9)

where the values of L are given in Table I and m$y be made
entirely negligible in the setting up of the apparatus.

TABLE I.—VALUES OF L

!IQ in centimeters

h' in centimeters

100
200
300

50
0.01
0.01
0.02

100
0.01
0.03
0.04

200
0.03
0.05
0.08

300
0.04
0.08
0.11