THE VISCOMETER OS

right angles to the axis of the tube, the dimensions of the capillary
may be most accurately ascertained. There is the further
advantage that other capillaries may be used without changing
the other constants of the apparatus. It is perhaps unneces-
sary to add that this instrument must be modified to make it
suitable for the measurement of gases.

The Most Suitable Dimensions for a Viscometer.—As we
shall see in Part II, it is more convenient to compare fluidities
than viscosities. Combining Eqs. (8) and (12) we obtain for
the general formula for the fluidity of all fluids

where R is the radius and I the length of the capillary in cen- \\

timeters, and V is the volume in cubic centimeters, all being lf'

reduced to the same temperature. No correction is necessary for ,J ft

changes in these dimensions later since the changes just neutralize !/J •

each other as can be proved by introducing the coefficient || s

of expansion into the above formula. The pressure p, expressed i^ .'

in grains per square centimeter, is the difference between the {|* ,

absolute pressures at the level of the capillary at the entrance, PI ^f »

and exit; P2. The time of flow in seconds is t} for the fluid whose *§ \

density is p at the temperature of observation T. T = 3.1416, (l ,'

g is the acceleration due to gravitation, m = 1.12, X is the j| j

coefficient of slipping, which is negligible for all liquids but is 1 \

of importance in rarefied gases, cf. page 244. In the measure- if i

ment of the fluidity of gases the volume of efflux must be cal- -| J
culated according to the formula of page 50.

T7 ^Pl T7 2.T2 TT

where Fi is the volume of flow as measured before flow under
pressure Pa and Y2 is the volume after expansion to the pressure
P*.
By the proper choice of the dimensions of the apparatus
an accuracy of one-tenth of 1 per cent may probably be attained.
With a stop-watch reading to 0.2 sec. the time of flow may be
made as small as 200 sec. The volume of flow should be small
for the following reasons. (1) The kinetic energy correction
mp V2 in Eq. (20) should be kept from becoming inconveniently