Upon another occasion the following rates and exhaustions were obtained:
Time. Exhaustion. Rate.
1/7,812,500
10 minutes }........ 1:1/3.18
1/24,875,620
10 minutes }........ 1:1/2.69
1/67,024,090
10 minutes }........ 1:1/1.22
1/81,760,810
10 minutes }........ 1:1.67
1/136,986,300
10 minutes }........ 1:1.23
1/170,648,500
The irregular variations in the rates are due to the mode in which the flow of the mercury was in each case regulated.
Leakage.--We come now to one of the most important elements in the production of high vacua. After the air is detached from the walls of the pump the leakage becomes and remains nearly constant. I give below a table of leakages, the pump being in each case in a condition suitable for the production of a very high vacuum:
Duration of the Leakage per hour in
experiment cubic mm., press.,
760 mm.
18½ hours............................ 0.000853
27 hours............................ 0.001565
26½ hours.............................0.000791
20 hours.............................0.000842
19 hours.............................0.000951
19 hours.............................0.001857
7 days..............................0.001700
7 days..............................0.001574
Average.................... 0.001266
I endeavored to locate this leakage, and proved that one-quarter of it is due to air that enters the gauge from the top of its column of mercury, thus:
Duration of the Gauge-leakage per hour
experiment. in cubic mm., press.
760 mm.
18 hours.................................0.0002299
7 days..................................0.0004093
7 days..................................0.0003464
Average.......................0.0003285
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.