The New Student's Reference Work/Air Pump
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Air Pump, an instrument used either to compress air in a closed vessel or to exhaust the air from a closed vesel. When used for the former purpose it is generally known as a "force pump," and when used for the latter purpose it is frequently called a "vacuum pump."
Air pumps, like other pumps, consist essentially of a cylinder fitted with a piston and two valves. The simplest of those which are used for compressing air is, perhaps, the ordinary bicycle pump illustrated in Fig. i. Here the piston is provided with a more or less flexible leather collar which allows the air to pass down around it as the piston is lifted. But on the down stroke of the piston this leather collar acts as packing and prevents the air from passing up. Hence this one part acts as both piston and valve.
A second valve, V2, at the bottom of the cylinder prevents the air in the tire from getting back into the cylinder during the upstroke of the piston.
The vacuum pump is built upon exactly the same principles as the force pump, only the direction in which the valves open is reversed.
The first artificial vacuum was produced by Otto von Guericke, about 1650, with a pump similar to an ordinary lift-pump used in wells, except that instead of pumping the air out of an open vessel, such as a well, he pumped it out of a closed vessel.
The immense improvement which has recently been made in the construction of air pumps, especially by the introduction of valves which open automatically, can be seen from the fact that Guericke's pump required four able-bodied men to operate it. A modern air pump is easily worked by one hand.
Ordinary vacua are thus produced by a piston working in a brass cylinder; but when it comes to "high vacua," such as those employed in the incandescent lamp and in X-ray bulbs, a much more perfect instrument is required. Here it is necessary to replace the sprengel air pump ordinary cylinder by a mercury column in a glass tube.
In Fig. 2 is given a diagram of the best of these mercury pumps, the one devised by Sprengel.
The supply of mercury is contained in the reservoir on the left. It flows over into the bulb B, where it falls in drops into the long tube on the right. These drops entrap between them the air in B. The mercury which runs out is collected and poured back into the reservoir on the left. In this manner practically all the air can be removed from the bulb B, and hence from any vessel R, which may be connected with B. At M is a manometer which indicates the pressure in the vessel R, which is being exhausted.
A pump of this type is capable of producing a vacuum in which the pressure is only 100,000,000th of an atmosphere.