It has been found that when the pump is in constant use the
gauge-correction gradually diminishes from day to day; in other words,
the air is gradually pumped out of the gauge-mercury. Thus on December
21, the amount of air entering with the mercury corresponded to an
exhaustion of
1/27,308,805 .......Dec. 21.
1/38,806,688 ...... Dec. 29.
1/78,125,000 .......Jan. 15.
1/83,333,333 .......Jan. 23
1/128,834,063 ......Feb. 1.
1/226,757,400 ..... Feb. 9.
1/232,828,800 ..... Feb. 19.
1/388,200,000 ......March 7.
That this diminution is not due to the air being gradually withdrawn
from the walls of the gauge or from the gauge-tube, is shown by the fact
that during its progress the pump was several times taken to pieces, and
the portions in question exposed to the atmosphere without affecting
the nature or extent of the change that was going on. I also made one
experiment which proves that the gauge-correction does not increase
sensibly, when the exhausted pump and gauge are allowed to stand unused
for twenty days.
_Rate of the pump's work_.--It is quite important to know the rate of
the pump at different degrees of exhaustion, for the purpose of enabling
the experimenter to produce a definite exhaustion with facility; also if
its maximum rate is known and the minimum rate of leakage, it becomes
possible to calculate the highest vacuum attainable with the instrument.
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