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. Examples are given in the tables below; the total capacity was about 100,000 cubic mm.

 Time. Exhaustion. Ratio. 
1/78,511 10 minutes }........ 1:1/3.53 1/276,980 10 minutes }........ 1:1/6.10 1/1,687,140 10 minutes }........ 1:1/4.15 1/7,002,000

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.

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.