Vacuum

The mercury vapour pumps, on account of their simplicity and rapidity, and of the high vacua which they produce, have obtained a foremost place among the weapons of the worker with high vacua, and are to be found not only in nearly all physical labora tories, but also in the workshops of the makers of X-ray tubes, thermionic valves, and the like. They have the great advantage that the speed of pumping is constant down to the lowest pres sures, and theoretically there is no limit to the degree of vacuum which can be produced, as there is with the molecular pump, where a given pressure ratio is established between high and low pressure side. Dozens of different modifications exist : in one form there is no external heating, the vaporization of the mercury being pro duced by striking a mercury arc within the pump itself. Some forms, like that of Dunoyer, work with relatively quite a high pres sure as fore-vacuum, but it is usual to employ some form of rotary box pump as fore-pump. Pressures as low as a few millionths of a mm. of mercury have been produced with these pumps. The speed of a single-stage pump, with a fore-vacuum of 1 mm. can be as high as an evacuation of I o,000 cc. per second.

To prevent mercury vapour from the pump reaching the ex hausted system, some form of trap is inserted between the two.

This may be a vessel of special form, offering a large surface to the vapour, cooled by liquid air, or vessels coated inside with an alkali metal, say sodium, may be used. The alkali metals exert an extraordinary absorption for mercury vapour.

Vacuum Production by Ab sorbers and Discharge.—To remove residual gases and va pours from a space already ex hausted by a pump and sealed off, use has been made of the strong absorption which purified charcoal exercises on gases when it is cooled. Sir James Dewar was the first to call attention to this technique. The charcoal is prepared from cocoanut shell (originally from the fleshy part of the nut) by prolonged heat ing in vacuo. A tube containing the charcoal is sealed onto the apparatus, and, after prelimin ary exhaustion has taken place, a vessel of liquid air is placed round the charcoal tube. Very low pressures can be produced in this way, and before the inven tion of the molecular pump and of the mercury vapour pump the method was extensively used in the laboratory, and still finds em ployment in certain types of ex periment. A great deal of work was done on the absorbing power of charcoal for gases in connec tion with gas masks during the war.

For particular gases and va pours other absorbers are some times used. Palladium black at low temperatures is particularly effective for hydrogen. The use of alkali metals for absorbing mercury vapour has received mention above. The liquid alloy of sodium and potassium is used for removing traces of oxygen, and other instances of substances used to take up residual gases by chemical action will be found in the standard treatises on high vacua.

For removing the residual gases from electric lamps, ther mionic valves, and the like, an operation to which the term "clean up" is technically applied, processes of a chemical nature are widely employed. A minute amount of some substance known as a "getter," which com bines vigorously with the gases, is introduced into the lamp, and the filament is then heated, when the clean up takes place. The process is described in the article ELECTRIC LAMPS AND VALVES, MANUFACTURE OF. The electric discharge in an evacuated lamp or similar vessel also leads to the disappearance of residual gases. The effect is a complicated one, which has been studied in detail by Norman Campbell and his co-workers in the laboratories of the General Electric Company, Limited.

Measurement of Low Pressures.--Pressures

down to a milli metre of mercury or so can be measured by an ordinary mer cury manometer, consisting of a U-tube of which one limb, closed at the upper end, is originally quite full of mercury. When the other is connected to the vessel in which the pressure is to be measured the difference of level in the two limbs, which may be read with a cathetometer, indicates the excess pressure in the ves sel above the vacuum in the closed limb. Such manometers are commonly used for rough readings. Lord Rayleigh (3rd Baron) devised a more sensitive mercury manometer, in which the two mercury levels are in contact with fine vertical pointers: the dif ference of pressure is measured by the angle through which it is necessary to tilt the manometer in order to restore the level when it is disturbed by the presence of gases on one side. This gauge has been used to measure pressures from I•S mm. to .00i mm. of mercury. Shrader and Ryder have described another way of measuring very small movements of the mercury surface in a U-tube manometer. A little float on the surface supports one end of a lever carrying a small mirror, and the tilt of the mirror is measured by a lamp and scale.

The McLeod gauge, described in the first part of this article is still a standard instrument for the accurate measurement of pres sures down to .001 mm. of mercury—or even lower, down to a hundredth of this value, with certain gases and suitable precau tions. The accuracy of the gauge depends upon the closeness with which the gas in question obeys Boyle's Law, which is assumed in the deduction of the pressure. It is clear that the gauge cannot be used with condensable vapours, such as those of water or ammonia, which tend to liquefy on the walls; even with carbon dioxide the readings are not reliable. With such gases as hydrogen and nitrogen, on the other hand, the gauge works down to the limit of pressure just mentioned.