314 FLUIDITY AND PLASTICITY
Publication of the temperature, time, pressures p and P, density,
viscosity, and fluidity makes the data quite complete and cor-
rection easy.
In constructing the viscometer, the glass blower must select
a piece of capillary tubing which has not only a uniform bore
but also one which has a radius which must be selected within
rather narrow limits. This requires the measurement of the
radius, which is accomplished as follows. The capillary is filled
with mercury completely to a distance of exactly 10 cm, this
mercury is then run out on to a watch crystal and weighed. The
radius of the capillary in centimeters can be read at once from
Table IX. These measurements need not be exact, but where it
is desired to measure the average radius with exactitude, as in
absolute measurement, it is to be noted that the volume of the
mercury is calculated for 20°C and that the values are corrected
for buoyancy of the air so that there is no correction in weighing
with platinum weights. It is assumed that the mercury thread
is a true cylinder.
Having found the radius of the capillary, it becomes feasible
to cut off a length which will give a time of flow of not less than
200 sec. for the assumed maximum fluidity, e.g., 500, with a
pressure of 50 g per square centimeter and a volume of flow of
4 ml. The lengths to be cut off for capillaries of different radii
are given in Table X (cf. also Fig. 24).
The table shows that with a maximum fluidity of 500 and a
permissible length of capillary up to 20 cm, the radius must not
be as great as 0.015 cm; and if the ratio of the length to the radius
is to be greater than 500 in order to minimize "end effects/' the
radius must be over 0.010 cm, which limits the selection within
quite narrow limits.
The viscometer with a 500 capillary will serve for quite viscous
liquids, for the pressure can be varied from 50 to 10,000 and the
time may conveniently be increased fivefold, hence one can
measure two-thousand-fold, i.e.} from 500 to 0.5. Nevertheless
in an investigation in which no fluidities are to be measured above
50, it is convenient to use a viscometer with a maximum of 50,
and therefore the length of capillary will be one-tenth of that
indicated by Table X. Just what maximum to specify, as 5,000
500, 50, 5, or 0.5, may easily be judged by the use of Tables XI