318 FLUIDITY AND PLASTICITY
only be at A when the right meniscus is at (?, but it should be at
B when the right meniscus is at J, and at D when the right menis-
cus is at L.
The ends of the capillary at E and F must not be contracted
in under any circumstances, and as far as practicable the ends
should not be expanded into a trumpet shaped opening, as this
may seriously affect the kinetic energy correction. The appro-
priateness of the correction already given may be tested for each
instrument by using a given liquid at a variety of pressures.
If the liquid is to move past the marks B and D with a
velocity of not less than 0.1 cm per second when the time of
flow is 200 seconds, it is only necessary to use tubing for
that part of the instrument whose radius is not more than
0.25 cm. On the other hand, for absolute measurements,
the instrument should always be so designed that the resist-
ance to flow outside of the capillary will be negligible.
This is ascertained as follows: Let the lengths of the tubes
B, D, L, and J, Fig. 1, be in all L and their radius be R.
Then J?4/L must be greater than 1,000 rVZ. For a capillary
whose radius is 0.012 cm, R must be at least 0.07 cm if L = 1.
This value is larger than is commonly supposed. A rule which
is simple but will cover every case is to have R at least 10 times
the radius of the capillary.