CHAPTER XV. AIR-PUMP. 129. Air-pump.-The air-pump was invented by Otto Guericke about 1650, and has since undergone some improvements in detail which have not altered the essential parts of its construction. It consists of a glass or metal cylinder called the barrel, in which a piston works. This piston has an opening through it which is closed at the lower end by a valve S opening upwards. The barrel communicates with a passage leading to the centre of a brass surface carefully polished, which is called the plate of the air-pump. The entrance to the passage is closed by a conical stopper S', at the extremity of a metal rod which passes through the piston-head, and works in it tightly, so as to be carried up and down with the motion of the piston. A catch at the upper part of the rod confines its motion within very narrow limits, and only permits the stopper to rise a small distance above the opening. Suppose now that the piston is at the bottom of the cylinder, and DEGREE OF EXHAUSTION. 185 is raised. The valve S' is opened, and the air of the receiver E rushes into the cylinder. On lowering the piston, the valve S' closes its opening, the air which has entered the cylinder cannot return into the receiver, and, on being compressed, raises the valve in the piston, and escapes into the air outside. On raising the piston again, a portion of the air remaining in the receiver will pass into the cylinder, whence it will escape on pushing down the piston, and so on. We see, then, that if this motion be continued, a fresh portion of the air in the receiver will be removed at each successive stroke. But as the quantity of air removed at each stroke is only a fraction of the quantity remaining, we can never produce a perfect vacuum, though we might approach as near to it as we pleased if this were the only obstacle. 130. Calculation of the Degree of Exhaustion.-It is easy to calculate the quantity of air left in the receiver after a given number of strokes of the piston. Let V be the volume of the cylinder, V′ that of the receiver, and M the mass of air in the receiver at first. On raising the piston, the air which occupied the volume V' occupies a volume V'+V; of the air thus expanded the volume V is removed, and the volume V' left, being of the whole quantity or mass M. V V' The quantity remaining after the second stroke is of that after V+V the first, or is () M; and after n strokes (v)"M. Hence the V + V density and (by Boyle's law) the pressure are each reduced by n strokes to () of their original values. + V We see, then, that the pressure goes on decreasing indefinitely, and that, consequently, the elasticity of the air may, theoretically at least, be rendered less than any assigned quantity. 131. Mercurial Gauge.-In order to follow the steps of the operation, and to observe at each instant the elastic force of the air in the receiver, the instrument is provided with a siphon-barometer, called the mercurial gauge, inclosed in a bell-shaped vessel of glass F, and communicating by a stop-cock with the receiver. This barometer consists of a bent tube, the branches of which are about a foot in length; one of these is closed and filled with mercury, the other is open. When the pressure of the air in the receiver becomes less than that represented by a column of mercury equal in length to the closed branch of the gauge, the mercury falls, and the elastic force of the air at any moment is given by the difference of level of the mercury in the two branches; this difference can be measured on a graduated scale. The mercurial gauge serves to show whether the instrument is working properly; for instance, in the case of air getting in anywhere, this would be shown by the fluctuations of the mercurial column. It also shows when the greatest possible effect has been attained, by the level of the mercury remaining stationary notwithstanding the motion of the piston. In theory, as we have said above, there is no limit to the action of the machine, and at each stroke of the piston the elastic force of the air should decrease; but in reality this is not the case, on account of the inevitable imperfections of the apparatus; there is always a limit, extending further in proportion to the excellence of the machine, and the barometer shows the moment when this limit is reached. Instead of a siphon-barometer, we might have an ordinary barometer in connection with the receiver, and thus observe the progress of the vacuum from the first strokes of the piston. 132. Admission Stop-cock.-After the receiver has been exhausted of air, if it was required to raise it from the plate, a very considerable force would be necessary, amounting to as many times fifteen pounds as the area of the plate contained square inches. It would, therefore, be in general impossible to raise the receiver. This is, however, rendered possible by means of the stop-cock R, which is shown in section above. It is perforated by a straight channel, which, when the machine is being worked, forms part of the communicating passage. At 90° from the extremities of this channel is another opening O, forming the mouth of a bent passage, leading to the external air. When we wish to admit the air into the receiver, we have only to turn the stopcock so as to bring the opening O to the side next the receiver; if, on the contrary, we turn it towards the pump-barrel, all communication between the pump and the receiver is stopped, the risk of air entering is diminished, and the vacuum remains good for a greater length of time. This precaution is taken when we wish to leave bodies in a vacuum for a considerable time. Another method is to employ a separate plate, which can be detached so as to leave the machine available for other purposes. 133. Double-barrelled Air-pump. The machine just described has only a single pump-barrel; air-pumps of this kind are sometimes employed, and are usually worked by a lever like a pump-handle. With this arrangement, it is evidently necessary that the piston, after having ascended, should descend again to expel the air from the DOUBLE-BARRELLED AIR-PUMP. 187 pump-barrel, and it is only after this double stroke that the operation can begin anew. Double-barrelled pumps are more frequently used. An idea of their general arrangement may be formed from Figs. 136, 137, and 138. Fig. 138 gives the machine in perspective, Fig. 136 is a section through the axes of the pump-barrels, and Fig. 137 shows the manner in which communication is established between the The piston-rods C are two racks working with the pinion P. This pinion is turned by a double-handed lever, which is worked alternately in opposite directions. In this arrangement, when one piston ascends the other descends, and consequently in each single stroke the air of the receiver passes into one or other pump-barrel. A vacuum is thus produced by half the number of strokes which would be required with a single-barrelled pump. It has besides another advantage. In the single-barrelled pump the force required to raise the piston increases as the exhaustion proceeds, and when it is nearly completed there is the resistance of almost an atmosphere to be overcome; that is, nearly 15 pounds to the square inch. In the double barrelled pump, at the moment when one piston is at the top, and the other at the bottom, the force opposing the ascent of the one is precisely equal to that assisting the descent of the other. We must observe, however, that this equality exists only at the beginning of the stroke; for when one of the pistons descends, the air below it is compressed, its tension becoming greater and greater, until it reaches that of the atmosphere and raises the piston-valve. At this moment the resistance to the ascent of the other piston is entirely uncompensated, and up to this point the compensation has been gradually diminishing. But the more nearly we approach to a perfect vacuum, the more slowly does the tension of the air compressed beneath the piston increase, so that, unlike the single-barrelled pump, it becomes easier to work as the exhaustion proceeds. 134. Single-barrelled Pumps with Double Action. We do not, however, require two pump-barrels in order to obtain double action, as the same effect may be obtained with a single barrel. An arrangement for this purpose was long ago suggested by Delahire for water |