76

FLUIDITY AND PLASTICITY

The pressure in this instrument must
proportional to the densities so that

be

PIG. 29.—Viscom-
eter suitable for the
relative me a sure-
merit of not too
Tiscous liquids.

=

which is the formula suggested by Ostwald.
The formula is true for dilute solutions when
water is taken as the standard, for fj is then
nearly equal to T?O.
It is inconvenient to make the time of flow
very large both on account of the lack of
economy and because of the increased danger
of clogging. Unfortunately this formula has
been used where neither of the necessary con-
ditions was complied with and the results are
therefore of uncertain value. It is much
better to make the correction for the kinetic
energy, in such cases, than to attempt to make
the correction negligible.
It is a disadvantage of the Ostwald instru-
ment that the pressure is not variable at will,
because if the time of flow is sufficient in one
liquid, in another more viscous liquid the time
of flow may be intolerably long, practically
necessitating the use of a variety of instru-
ments. Furthermore the total pressure is so
small that a, small error in the working volume
may introduce considerable error into the
result and the density of the liquid must be
known with considerable accuracy.
A form of instrument which has the mani-
fest advantages of the Ostwald instrument
and overcomes the above objections is shown
in Fig. 29. The volume K is made as nearly
as possible equal in volume, similar in shape,
and at the same height as C. The working
volume is contained between A and H and
the volume of flow between B and D, the
measurement being made as the meniscus
passes either from B to D or from D to -B