APPENDIX B 323

of mercury are required. Were a smaller tube used for the outer
tube of the stabilizer, less mercury would be required but the
manipulation might be less convenient. A photograph of the
plastoraeter occording to the latest design used by Mr. H. D.
Bruce is reproduced in the frontispiece.

TREATMENT or PLASTICITY DATA

The data may be analyzed either algebraically or graphically.
The formula for plastic flow through a capillary tube is
1 _ irRH mpV

M~ 4

where ^ is the mobility, and / the friction or yield value. The

7?P/y

shearing force, F — ——-, is expressed in dynes per square centi-

meter and the pressure P is expressed in grams per square centi-
meter. Since the kinetic energy is generally negligible this
becomes

Kv /1Q\

A* = j—j (13)

where v is the volume of flow per second and K is a constant whose

R*
value is 384.8 -y. If we substitute in Eq. (13) the values

Fiy vi and F^ v<> from two observations of the flow, we find that

=

so that both ja and / are readily determined. Since however the
weight of flow w = vp, a more convenient expression for the
friction is

-
wz — Wi
The friction must have a positive value for all plastic substances
and the value should be constant for a given capillary so long as
seepage, slipping, et cet.} do not intervene.
In the early stages of the development of the subject, the
graphical method of treatment is desirable from many points of
view. Plotting the weight of flow in grams per second as ordi-
nates and the shear in dynes per square centimeter as abscissas,
the value of the intercept of the extrapolated curve gives the value
of the friction and the slope of the curve determines the mobility.
The curvature indicates to what degree seepage, et cet., enter in.