306

FLUIDITY AND PLASTICITY

tage of the drain pipe U is that of securing day by day practically
identical pressures, without the loss of time in adjustment. If
other pressures than these are desired, they may be obtained by
drawing off some of the water from one or more of the stand pipes.
The glass gage at J', etc., aid the manipulator in adjusting the cur-
rent of air. They may be cleaned by unscrewing the pet-cocks
above and using a small brush.
The beginner must be cautioned particularly against turning
the system to air at the viscometer since it may result in filling
the manometer, etc. with water. To prevent such an accident
and to dry the air, the reservoir P containing granular calcium
chloride is introduced. Any liquid should be drained at intervals.
THE MANOMETER
The manometer consists of a plate glass mirror which must be
mounted vertically, on which is stretched a 2-m steel tape
graduated in millimeters. Over the tape is fixed the glass tube
of the manometer bent so that both the right and left limbs may
be read on the same tape. The manometer may be filled with
either mercury or water. If water is used for low pressures
another manometer will be desired for mercury. Since it is
possible to read the manometer to 0.01 cm one can use the mer-
cury manometer down to 10 cm (135 g per square centimeter)
with the desired accuracy. With water one can go down to
about 50 g per square centimeter, but not much further unless a
correction is made for the true average pressure. A thermometer
near the middle of the manometer is needed to give the tem-
perature of the manometer fluid.
THE BATH
The viscometer V is mounted on a massive brass frame Fig.
93 by means of brass clips designed especially for this purpose.
The frame slides in grooves on the side of the bath so that the
viscometer may be easily kept in a vertical position. The viscom-
eter is connected by heavy-walled rubber tubing to the pressure
by way of the three-way glass stop cocks L and R, the third
connection being to air. The temperature of the bath is raised
by means of a burner W which is connected without the use of
rubber to the gas supply. The second burner Y with stop cock
and pilot flame is used as needed to obtain the fine regulation.