74

FLUIDITY AND PLASTICITY

be made. (1) The pressure on the manometer must be calculated
to grams per square centimeter from the known height of the
liquid and its specific gravity at the temperature observed. A
correction to the observed height of the liquid is avoided by
having the long limb of the manometer doubly bent at its middle
point so that the upper half is vertical and in the same straight
line with the lower limb of the manometer. The levels on both
limbs may then be read on the same scale, which may con-
veniently consist of a steel tape mounted on a strip of plate-glass
mirror placed vertically. Similarly a correction for capillary
action may be avoided if the bore of the manometer is large
enough so that it may be assumed to be uniform. (2) The pres-
sure must be corrected for the weight of the air displaced by the
liquid in the manometer. (3) "Unless the surface of the liquid in
the lower limb of the manometer is at the same height as the
average level of the liquid in the viscometer, a correction must be
made for the greater density of this enclosed air, than of the
outside air which is not under pressure. (4) Finally a correction
must be made for the average resultant hydrostatic head of the
liquid within the viscometer. If the two volumes V and V
in Fig. 23 are exactly equal in volume, similar in shape, and
at the same elevation above the capillary, when the viscometer is
in position, in the bath, it is evident that the gain in head during
the first half of the flow will be exactly neutralized by the loss
in head during the last half of the flow. Since this cannot be
exactly realized, a correction may be made as follows: Duplicate
observations in reverse directions are made upon a liquid of
known density and viscosity at a constant temperature and
pressure. Let ti be the time of flow from left to right and U the
corresponding time from right to left. Let p0 be the pressure as
corrected, except for the average resultant head of liquid in the
viscometer. Suppose this latter correction to amount to x em
of the liquid as the liquid flows from left to right. In this case
the total pressure becomes equal to p0 4- P% and when the
liquid flows from right to left, it becomes equal to pQ — px.
Since Eq. (8) when used for a given viscometer may be written
in the form
, = Opt - C'p/t (22)