VISCOSITY AND FLUIDITY

or disk type, we actually measure the velocity v, BS in the figure,
and we very naturally assume that

P =

ffv
R

2. It is more usual, however, to calculate the viscosity from
the volume of flow, as in the Poiseuille type of instrument.
let vf, £S' in the figure, be the effective velocity which the
surface JBS would have, were the series of lamellae replaced by

S' S 5"

FIG. 33.—Diagram to illustrate additive fluidities.

a homogeneous fluid having the same volume of flow. The
effective velocity is related to the quantity of fluid U passing per
second in a stream of unit width, as follows:

Let the viscosity as calculated from the flow, as for a homo-
geneous fluid, be H', then

•flV ZH'U

P =

E

(24b)

It is to be noted that had the less viscous substance been in
contact with the surface AE, the effective velocity of flow would
have been represented by the distance BS'7. We shall take the
former of these for our definition of the viscosity of a mixture,