This renders it very probable that the remaining three quarters are due
to air given off from the mercury at B, Fig. 4, from that in the bends
and at the entrance of the fall-tube, _o_, Fig. 3.
Further on some evidence will be given that renders it probable that the
leakage of the pump when in action is about four times as great as the
total leakage in a state of rest.
The gauge, when arranged for measurement of gauge-leakage, really
constitutes a barometer, and a calculation shows that the leakage would
amount to 2.877 cubic millimeters per year, press. 760 mm. If this air
were contained in a cylinder 90 mm. long and 15 mm. in diameter it would
exert a pressure of 0.14 mm. To this I may add that in one experiment
I allowed the gauge for seven days to remain completely filled with
mercury and then measured the leakage into it. This was such as would
in a year amount to 0.488 cubic millimeter, press. 760 mm., and in a
cylinder of the above dimensions would exert a pressure of 0.0233 mm.
_Reliability of the results: highest vacuum._
The following are samples of the results obtained. In one case sixteen
readings were taken in groups of four with the following result:
Exhaustion.
1 / 74,219,139
1 / 78,533,454
1 / 79,017,272
1 / 68,503,182
Mean 1 / 74,853,449
Calculating the probable error of the mean with reference to the above
four results it is found to be 2.28 per cent of the quantity involved.
A higher vacuum measured in the same way gave the following results:
1 / 146,198,800
1 / 175,131,300
1 / 204,081,600
1 / 201,207,200
The mean is 1 / 178,411,934, with a probable error of 5.42 per cent of
the quantity involved. I give now an extreme case; only five single
readings were taken; these corresponded to the following exhaustions:
1 / 379,219,500
1 / 371,057,265
1 / 250,941,040
1 / 424,088,232
1 / 691,082,540
The mean value is 1 / 381,100,000, with a probable error of 10.36 per
cent of the quantity involved. Upon other occasions I have obtained
exhaustions of 1 / 373,134,000 and 1 / 388,200,000. Of course in these
cases a gauge-correction was applied; the highest vacuum that I have
ever obtained irrespective of a gauge-correction was 1 / 190,392,150. In
these cases and in general, potash was employed as the drying material;
I have found it practical, however, to attain vacua as high as 1 /
50,000,000 in the total absence of all such substances. The vapor of
water which collects in bends must be removed from time to time with a
Bunsen burner while the pump is in action.
It is evident that the final condition of the pump is reached when
as much air leaks in per unit of time as can be removed in the same
interval. The total average leakage per ten minutes in the pump used by
me, when at rest, was 0.000211 cubic millimeter at press. 760 mm. Let
us assume that the leakage when the pump is in action is four times
as great as when at rest; then in each ten minutes 0.000844 cubic
millimeter press., 760 mm., would enter; this corresponds in the pump
used by me to an exhaustion of 1 / 124,000,000; if the rate of the pump
is such as to remove one-half of the air present in ten minutes, then
the highest attainable exhaustion would be 1 / 248,000,000. In the same
way it may be shown that if six minutes are required for the removal of
half the air the highest vacuum would be 1 / 413,000,000 nearly, and
rates even higher than this have been observed in my experiments. An
arrangement of the vacuum-bulb whereby the entering drops of mercury
would be exposed to the vacuum in an isolated condition for a somewhat
longer time would doubtless enable the experimenter to obtain
considerably higher vacua than those above given.
_Exhaustion obtained with a plain Sprengel Pump._--I made a series of
experiments with a plain Sprengel pump without stopcocks, and arranged,
as far as possible, like the instrument just described. The leakage per
hour was as follows:
Duration of the Leakage per hour in
experiment. cubic mm. at press.
760 mm.
22 hours 0.04563
2 days 0.04520
2 days 0.09210
4 days 0.06428
-------
Mean 0.06180