THE LAW OP POISEUILLE

9

FIG. 1.—Poiseuille's viscomo-
ter.

water or mercury, and (3) a pump which is used for giving the
desired pressure. The capillary opens into the distilled water
of the bath in which the .bulb and capillary are immersed. After
the dimensions of the bulb and capillary have been found, it is
only necessary, in making a viscosity determination at any given
temperature, to observe the time necessary for a volume of
liquid equal to that contained in the bulb to flow through the
capillary under a determined pressure. Without going into detail
at this point, it need be merely stated
here that due means were taken for
getting the true dimensions of the
capillary and bulb, for filling the
apparatus with clean pure liquid, and
for estimating the mean effective pres-
sure, which consists of the pressure
obtained from the manometer plus
the hydrostatic pressure from the
bottom of the falling meniscus in the
bulb to the level of the capillary, minus the hydrostatic pres-
sure from the level of the capillary to the surface of the bath,
minus a correction for the capillary action in the bulb, and two
corrections for the pressure of the atmosphere, which may be
either positive or negative. One of these last corrections is due
to the air within the apparatus being more dense than that
outside, the other is due to the difference of pressure of the atmo-
sphere upon the liquid surfaces in the upper arm of the manom-
eter and in the bath, unless they happen to be at the same level.
Law of Pressures.—In obtaining this law all of the experi-
ments were made at a temperature of 10°C. For a capillary of
given length and diameter, the time of transpiration was meas-
ured for various pressures. For example, one capillary was 75.8
mm long, the major and minor axes of the end of the capillary
nearer the bulb were 0.1405 and 0.1430 mm and those of the
open end 0.1400 and 0.1420 mm respectively. The pressures
used are given in the first column of Table I and the times of
transpiration in column 2. One of these values is then employed
to calculate the others on the assumption that the times of tran-
spiration are inversely proportional to the pressures, as given in
column 3.