APPENDIX A 303
The correction for the difference in level between the middle
of the manometer and the viscometer is made negligible in setting
up the apparatus.
MEASUREMENT OF TIME
We have seen that the pressure in grams per square centimeter
must always be 30 times as great as the distance between the
bulbs. On the other hand the pressure must always be kept small
enough so that the time of flow can be measured to the desired
accuracy. Thus the time should not fall below 200 sec. since one
cannot measure the time more accurately than to 0.2 sec. with a
stop-watch.
The stop-watch should be tested repeatedly against the second
hand of a good time piece. It should not gain or lose as much as
0.2 sec. in 5 min. It is well to keep the watch in the same posi-
tion during successive measurements, as well as not to allow it to
be nearly run down during a measurement. In selecting a stop-
watch it should be noted that watches show better performance
whose mechanism continues to run whether the split-second hand
is in use or not. The performance of the watch may be tested
at the U. S. Bureau of Standards.
TEMPERATURE
The viscometer is kept at a constant temperature by means of a
large, well-stirred bath which is regulated by hand, if a series of
temperatures are to be measured, or by a thermostat, if the bath
is to be used for a long time at a single temperature. Since at
0° the fluidity of water increases 0.1 per cent for every 0.03°
rise in temperature it is clear that the temperature regulation
must be to at least 0.03°. For more viscous substances a still
more precise regulation is necessary if the same degree of accu-
racy is to be obtained.
A thermometer should be used which is graduaded to tenths
and calibrated through its entire length. The ice point should
be determined from time to time. If it is impracticable to have
the entire thread of mercury immersed at all times a correction
should be made for the emergent stem. The following table
may be used: