304

FLUIDITY AND PLASTICITY

TABLE V.—CORRECTION OF A NORMAL THERMOMETER FROM 0° TO 100°C
FOR EMERGENT STEAM GRADUATED IN TENTHS OF A DEGREE

Number of de-
grees of mercury

Difference in temperature between mean temperature

of emergent steam and bath. Corrections in degrees to be

added to the observed temperature

exposed
30°
40°
50°
60°
70°
80°

10
0.05
0.05
0.05
0.05
0.10
0.10

20
0.10
0.15
0.15
0.15
0.20
0.20

30
0.20
0.25
0.25
0.25
0.30
0.35

40
0.30
0.30
0.35
* 0.40
0.45
0.50

50
0.35
0.40
0.45
0.50
0.55
0.60

Since measurements are always preferred for even degrees it is
a great advantage for the worker to have on the bath before him
a table showing what temperatures on the thermometer must be
employed in order to obtain a desired even temperature. The
temperatures of 0°, 10°, 20°, 40°, 60°, 80°, 100° are sufficient to
give a good curve over this range.
THE PRESSURE REGULATOR
Viscosity measurements have usually been carried out without
the use of a pressure regulator, but due to the withdrawal of the
air in use and to possible small leaks in the connections and to
changes in temperature, the pressure rises and falls and is hardly
ever constant during the time of a single measurement. With
a pressure regulator the pressure will often stay constant to the
limit of the experimental error for a day or more at a time, with-
out temperature regulation of the room, heat insulation of the
apparatus or any particular care in using the air. Not only is
this a saving of time and annoyance to the experimenter but by
using only a few pressures at the most there is a considerable
saving of time in calculation. Hence the pressure regulator
is a necessity for extended work.
The diagrammatic view of the apparatus with pressure regula-
tor is given in Fig. 92. Air is forced in through a needle valve A
to a storage reservoir B whose pressure in pounds per square inch
is shown on the gauge C. In adjusting the pressure regulator
the air is very slowly admitted to the stabilizing reservoir F by