70

FLUIDITY AND PLASTICITY

instrument. Finally the small bulbs B, E, Bf, and E1 should
have nearly the same volume. By having the surfaces nowhere
depart greatly from the vertical, the drainage is improved. It
is impracticable however to use long, cylindrical bulbs, since
then the true average pressure, due to the hydrostatic head within
the instrument, becomes awkward to determine. (Cf. Appendix
A, page 298.) The best form for the bulbs V and V is therefore
obtained by making them so that each resembles as much as

006 .009 .010 .Oil .012 0.5 ^0.4- t 0.15 JDIG .017 -015 .019
Radius in <

.020 -Oei

FIG. 24.—Chart for use of instrument maker in selecting capillary for vis-
cometer, knowing the approximate radius of the capillary and the maximum
fluidity to be measured, the length to be used may be read off. V =3ml t —200
sec,, p =50 g per cm2.
possible a pair of hollow cones, placed base to base as shown in
Fig. 23.
The marks at I and I1 are so placed that the volume from I to
Ff is exactly equal to that from V to F. If the two lirnbs of the
apparatus are similar there will be no correction for capillarity.
Poiseuille has givea a method for estimating this correction when
that is necessary. The volumes V and V may be easily deter-
mined by the weight of volumes of mercury.
The appearance of the complete apparatus used by Thorpe
and Rodger is shown in Fig. 25. The viscometer is shown in the
. hath B which has transparent sides. Water in the vessel R
exerts pressure upon the air in the large reservoir L. The gas