Vacuum

VACUUM (from Latin vacuus meaning empty), a space containing a highly rarefied gas. The simplest vacuum can be obtained by the method first used by Torricelli, for which it is sufficient to fill a tube sealed at one end, the length of which is less than 78–80 cm, with mercury, and invert its open end into a dish of mercury; then the mercury level will drop so that the remaining column will exert a pressure equal to atmospheric pressure; above the surface of the mercury there will remain the so-called "Torricellian vacuum." In Torricelli's time, it was indeed believed that a void was formed between the end of the mercury column and the upper walls of the tube. But in reality, mercury vapor and sometimes various impurities must inevitably be present in the resulting space if the mercury is not entirely pure. Torricelli's principle was laid at the foundation of mercury pumps, which at the present time have almost fallen out of use. Another old form of apparatus serving to produce a vacuum is piston pumps. Among modern pumps, the so-called Gaede oil pumps are most frequently applied. To obtain a higher vacuum, the extremely simple and ingeniously designed pumps of Langmuir are often employed. In these, mercury is heated in a reservoir by means of a gas or electric heater, causing it to boil (at low pressure). Mercury vapors pass through a constricted tube and are subsequently condensed in a cooler (after which they flow back into the reservoir). Along their path, the mercury molecules entrain gas molecules coming from the space being evacuated. For the reliable operation of the pump, it is necessary to continuously remove these molecules by creating the so-called preliminary vacuum (or fore-vacuum) in front of the Langmuir pump. For this latter purpose, the Gaede oil pump described above often serves. Measurement of vacuum. At the present time, the McLeod gauge is frequently used to measure high vacuums. The space being investigated is connected to a tube, to the bottom of which is attached a rubber tube filled with mercury (with a funnel-shaped reservoir at the end). To the side of it adjoins an appendage with a bulb and a thin capillary tube on the latter. As long as all the indicated branches communicate with one another, the pressure inside the bulb and capillary is obviously equal to the gas pressure in the space being investigated, i.e., to some hitherto unknown value. If the reservoir with mercury is raised, the communication between the bulb and the rest of the tube system will first be interrupted, and then the gas in the bulb and capillary will begin to be compressed. It can be compressed to such an extent that the mercury will enter the capillary and settle there at a certain level. If it is known that the remaining volume of gas constitutes 1/n of the volume of the bulb together with the capillary, then, as can be proven, the desired pressure must be expressed as 1/n of the height of the mercury column. Thus, the reading accuracy is increased by a factor of n, which is extremely important at strong rarefactions. Vacuum tubes. Tubes with rarefied gases inside which an electric discharge takes place are used chiefly for spectroscopic and spectrographic work. Especially remarkable are the new tubes in which the discharge occurs at relatively small potential differences: they are known under the name of neon and argon lamps. The energy source in them is not the induction coil feeding Geissler and Plücker tubes, but the city lighting network. Disinfection vacuum apparatus, see Disinfection apparatus.