The elasticity of gases differs from that of solids in that it is elasticity of volume and not of form. Gases are said to be perfectly elastic because they have no elastic limit. (§ 17); but their elasticity is not very great as measured by Formula 1. 159. The Air Pump is an instrument used for removing the air from any vessel with which it is connected. The possibility of doing this depends entirely upon the fact that air is elastic. A simple air pump consists of a cylinder A (Fig. 139), in which moves a piston attached to the rod B. A tube C connects this cylinder with a receiver D, from which the air is to be removed. There are two valves between the air in D and the external air: one in the base of the cylinder, and the other in the piston. These open upward, allowing the air to move in one direction only. At E there is a stopcock so arranged that it will permit free passage for the air between the pump and the receiver, or cut the receiver off altogether. F is an air-tight glass tube which communicates with the receiver, and contains a closed manometer (see § 179, b) for measuring the degree of exhaustion. 160. The Operation of the Pump is as follows: Suppose the piston to be at the top of its stroke. The first movement downward will slightly compress the air under it. This closes the valve at the base of cylinder A and opens the valve in the piston. The air passes through this valve, and when the piston is at the bottom of its stroke, the air in the cylinder is above it. As soon as the piston is raised, the pressure below it is decreased and the valve is closed, and as the piston rises, the air is forced out of a hole in the top of the cylinder, or through the small tube G. The cylinder below the piston is filled with air expanding from D. Each stroke is only a repetition of the first, except that the amount of air taken out diminishes with every stroke. 161. The Fleuss Pump. One of the most satisfactory mechanical air pumps is the Fleuss or Geryk pump, a diagram of which is shown in Fig. 140, while Fig. 141 shows the pump itself. The cylinder is separated into two compartments by a diaphragm D. There is an opening in this diaphragm, which is closed by a collar C, through which passes the piston rod, forming an airtight joint. The collar is kept down by the pressure of a spring S, and the lower part of each compartment is filled with a heavy oil. When the piston is raised, the valve V FIG. 140 closes, the oil above it is raised and the air compressed, until the shoulder on the piston strikes C and raises it, when the air and a part of the oil pass through into the upper chamber. When the piston is lowered, a part of the oil runs back to the lower compartment, but the air, being above the oil, cannot run through. The chamber B is connected to the space from which the air is to be exhausted. The tube T connects B with the cylinder below the piston, thus preventing the formation of a vacuum there when the piston rises. The oil makes the valves air-tight, so that this pump is easy to work and of high efficiency. FIG. 141. Fleuss Pump 66 162. Uses of the Air Pump. There are many practical uses of the air pump, among them its application to “ vacuum pans" for the making of sugar, in which the pressure is kept low, so that evaporation will take place at a lower temperature and the sugar will not be burned. Another important use of the air pump is in the manufacture of incandescent lamps and thermos bottles. FIG. 142 163. The Condensing Pump.-If the valves in the pump shown in Fig. 139 were arranged to open downward instead of upward, the P pump would be a condenser. When it is necessary, however, to transfer a gas from one vessel to another, an arrangement like that shown in Fig. 142 is used. In this pump the piston head has no valve. The pipe P is attached to the gas supply, FIG. 143 and the valve in P opens toward the cylinder. The pipe P' is attached to the vessel in which the gas is to be compressed, and its valve opens away from the cylinder. The valveless bicycle pump, Fig. 143, consists of a tube, to one end of which there is fixed a piston head carrying a concave leather collar. When the cylinder is drawn back, the air passes around the collar, but when it is pushed forward the collar fills the cylinder, the air is compressed, and passes through the tube into the tire. 164. Uses of Compressed Air.-Pascal's Law of the equal transmission of liquid pressure may be applied to gases as well; hence compressed air may be applied to the transmission of power. There are many machines in which a practical application of such transmission is used. Riveting hammers are used for forming the rivet heads on steel work. Pneumatic tools are also used in stone cutting, calking the seams 4, piston; B, hammer; C, trigger for pipes, iron chipping, drilling, and the like. Rock drills, sand blasts for cleaning metal surfaces, railroad signal |