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.