Physics Part 13

=60. The Air Pump.=--The air pump is used to remove air or other gases from a closed vessel. It was invented about 1650 by Otto Von Guericke, burgomaster of Magdeburg, Germany. One form of air pump is shown in Fig. 39. _C_ is a cylinder within which slides a tightly fitting piston.

_R_ is the vessel from which the air is to be exhausted. _r_ and _u_ are valves opening upward. The action of the pump is as follows:

On pushing the piston down, the air in _C_ is compressed. This opens valve _r_ allowing the confined air to escape above the piston. The piston is then raised making the s.p.a.ce in _C_ a partial vacuum. The pressure in _R_ now being greater than in _C_, _u_ is pushed up and the air from _R_ rushes into _C_, until the pressure is equalized. On pushing down the piston again, valve _u_ closes and the process is repeated until the pressure in _R_ is no longer able to raise the valve _u_. Some air pumps are so constructed that the valves are opened and closed automatically by the movement of the piston. With these pumps a higher degree of rarefaction can be obtained.

Air is often partially exhausted from receivers or vessels by the use of a filter pump or _aspirator_. A stream of water flowing through a constriction causes a reduced pressure, draws in air and carries it away, and thus produces a partial vacuum. See Fig. 40 for a section of the device.

[Ill.u.s.tration: FIG. 40.--An aspirator.]

=61. The Condensing Pump.=--This is like the exhaust pump except that its valves are reversed. It is used in compressing illuminating gases into cylinders for use in lighting vehicles, stereopticons, Pintsch lights, gas light buoys, etc., and also for compressing air to operate air brakes, pneumatic hammers and drills, and for other uses.

The common condensing pump is the kind used for inflating tires. (See Fig. 41.) In this, a loosely fitting metal piston is attached to a disc of leather somewhat larger than the cylinder. This device is called a _cup valve_. On raising the piston, air rushes in from the top past the valve, but on pushing the piston down, the valve is pressed tightly against the sides of the cylinder and prevents the escape of any air.

The compressed air pushes open a valve on the tire and enters it. This valve closes as soon as the pressure is lessened from outside. It is well to notice in all of these pumps that _two_ valves are used. One holds the air already secured while the other opens for a new supply.

Both valves are never open at the same time.

[Ill.u.s.tration: FIG. 41.--Condensing pump used in inflating tires.]

=62. Water Pumps.--The Common Lift Pump.= This, the simplest pump for raising water, consists of a cylinder _C_ (Fig. 42) connected by a pipe _R_ to a supply of water as a cistern or well. A valve opening upward is placed at the bottom of the cylinder over the entrance to the pipe. In the cylinder is a tightly fitting piston connected by a rod to a lever for ease in action. The piston contains a valve opening upward. In operating this pump water is usually first poured into the cylinder to "prime" it. This helps to close the valves and prevents air leaking past them. When the piston is lowered the lower valve closes, the air in the cylinder being compressed pushes the upper valve open and pa.s.ses above the piston. On raising the piston the upper valve closes. This forms a partial vacuum in the cylinder.

The air pressing on the surface of the water below forces the water and air that may be in the tube upward through the lower valve to fill this partial vacuum.

When the cylinder becomes filled with water, this is lifted out on the up-stroke, whence its name, "lift pump." Since the atmospheric pressure at sea-level can only support a column of water about 34 ft. high, the lower valve must be within this distance of the water surface. In actual practice the limit is about 27 ft. In deeper wells, the cylinder and valves are placed so that they are within 25 or 27 ft. of the surface of the water in the well, a long piston rod reaching above the surface of the ground and connected to a pump handle operates the piston. A discharge pipe extends from the cylinder to the surface of the ground above.

[Ill.u.s.tration: FIG. 42.--The common lift pump.]

[Ill.u.s.tration: FIG. 43.--A force pump with an air chamber (_A_).]

=63. The Force Pump.=--The force pump is used to deliver water under pressure either for spraying or to an elevated reservoir. The piston is solid, the second valve being placed at the entrance of the discharge pipe. (See Fig. 43.) The action is the same as that of the lift pump, with this exception; the piston in its down stroke forces the water out through the discharge pipe, the velocity depending upon the pressure exerted.

A force pump is usually provided with an air chamber which is connected with the discharge pipe. On the down stroke of the piston, water is forced into the air chamber. This compresses the air it contains. The compressed air reacts and exerts pressure on the water forcing it out in a steady stream.

Force pumps are used in deep wells, being placed at the bottom.

The pumps used in city water works, fire engines, and all steam pumps, are force pumps. (See Fig. 44.)

[Ill.u.s.tration: FIG. 44.--A steam pump used on a fire engine.]

=64. The Siphon.=--The siphon is a tube used to convey a liquid from one level over an elevation to a lower level by atmospheric pressure. It is used to remove liquids from tanks or vessels that have no opening at the bottom.

The siphon cannot be completely understood until one has mastered the laws of the flow of liquids. The following is offered as an incomplete explanation of its behavior. Consider the siphon to be full of water and closed at _d_ (Fig. 45). Atmospheric pressure on _a_ will hold the siphon full if _ab_ does not exceed 34 feet. If _d_ is opened the water falls out with a speed equal to that acquired in falling from the level of _a_ to that of _d_. This speed is acquired by all the water in the siphon and results in a drop in pressure throughout it. The pressure at _a_ inside the siphon becomes less than the pressure at the same level outside as soon as the water starts flowing. The water in the vessel then flows into the siphon and out at _d_. This flow continues as long as there is a fall from the free surface of the water in the vessel to the outlet at _d_.

[Ill.u.s.tration: FIG. 45.--Cross-section of a siphon.]

[Ill.u.s.tration: FIG. 46.--The Cartesian diver.]

=65. The Cartesian Diver.=--This is a device which ill.u.s.trates at the same time transmission of pressure by liquids, Archimedes' principle, and compressibility of gases. It was invented by Des Cartes (1596-1650).

As ordinarily made, it is a hollow gla.s.s image with a small opening in the foot. It contains air and water in such amounts that the average density of image and contents is slightly less than that of water. It is placed in a tall gla.s.s jar filled with water and covered with tightly stretched rubber tissue. (See Fig. 46.) By pressing on the rubber cover the diver may be made to sink, since the air and water transmit the pressure on the cover which compresses the air inside the figure admitting some water to it, thus making the diver more dense than water. By varying the pressure it can be made to sink, rise, or remain stationary at will.[D] A small vial can be used instead of the image.

[D] The position of a submarine in or under water is controlled in a similar manner.

=66. Hydraulic Ram.=--The hydraulic ram (see Fig. 47) is an automatic device that is much used for raising water from springs to houses located on higher ground. Water flows through the pipe _A_ through the opening at _B_. The pressure closes the valve at _B_. The increased pressure in the pipe due to the closing of _B_ opens the valve _C_ and some of the water flows into the air chamber _D_. This reduces the pressure against the valve _B_ so that it drops and allows a little water to escape. Just as this happens, valve _C_ closes. The pressure in the pipe then closes _B_ and forces water past _C_. This action being continually repeated, the air in _D_ becomes so compressed that it has elastic force enough to raise the water in a steady stream to a height of many feet.

[Ill.u.s.tration: FIG. 47.--Cross-section of a hydraulic ram.]

=67. The Balloon.=--Since air is a fluid, Archimedes' principle applies to it as well as to liquids. Therefore any object in the air is lifted up by a force equal to the weight of the air it displaces. The object will rise, if it weighs less than this displaced air and will continue to rise until both weights are equal.

_The Balloon_ (Fig. 48) rises because it weighs less than the air it displaces, and therefore it is pushed up by the heavier air, the "lifting power" being the difference between its weight and that of the air displaced. The neck at the bottom is left open to allow for expansion of the gas. When the aeronaut wishes to descend, he opens a valve at the top allowing some of the gas to escape.

[Ill.u.s.tration: FIG. 48.--Winner of international championship race, Paris, 1913.]

_Hydrogen_ is the lightest gas, weighing 0.09 kg. per cubic meter, and so gives the greatest lifting power, but as it is expensive to make, coal gas, density 0.75 kg. per cubic meter, is ordinarily employed.

Helium has recently been used to fill military balloons because it cannot be set on fire.

_The Parachute_ (Fig. 49) is an umbrella-shaped device for use in descending from a balloon. After falling a few seconds it opens, the large surface exposed to the air causing it to descend slowly. The hole in the top keeps the parachute upright by allowing the air to escape through it, thus relieving the pressure.

[Ill.u.s.tration: FIG. 49.--A parachute.]

[Ill.u.s.tration: FIG. 50.--Cross-section of a Westinghouse air brake.]

=68. The Air Brake.=--Compressed air is used to do work in many machines, such as pneumatic drills, hammers, and air brakes. The Westinghouse air brake (Fig. 50) uses air at a pressure of about 70 lbs.

to the square inch. The essential parts as shown are a reservoir _R_, the brake cylinder _C_ and a triple valve _V_, placed under each car with an air pipe _P_, leading to the engine. This is connected to _R_ by the triple valve _V_. When the pressure in _P_ is reduced by the engineer or by accident, the triple valve operates so as to admit air from _R_ into the cylinder _C_ pushing the piston _H_ to the left. _H_ is connected to the brakes by levers which press the brake shoes strongly against the wheels. When the air pressure in _P_ is restored the triple valve acts so as to permit the air in _C_ to escape while _R_ is filled again from _P_. The hissing sound heard when a train stops is caused by air escaping from cylinder _C_. The spring in _C_ keeps the brakes from the wheels except when the "air is on."

[Ill.u.s.tration: FIG. 51.--Cross-section of a gas meter showing its construction and action.]

=69. The Gas Meter.=--The gas meter consists of a box divided into two parts by a vertical part.i.tion (Fig. 51). Two bellows are attached to this part.i.tion, one on each side. The valves that regulate the flow of gas to and from the bellows and the chambers _A_ and _D_ are opened and closed by levers connected with the bellows. These levers also operate the hands upon the dials. When the inlet to the bellows _B_ is opened, the outlet of _A_ is also opened. Gas entering _B_ opens the bellows and forces the gas in _A_ out into the house-pipe _E_. When _B_ is full its inlet valve closes and its outlet valve opens. The inlet of _A_ also opens and its outlet closes. Gas now flows into _A_, compressing the bellows and _B_, and forcing the gas from it into the house-pipe. At each filling of the bellows _B_ there will be displaced from _A_ and forced into the house-pipe as much gas as enters _B_. It is evident that at each emptying of _B_ an equal amount of gas enters _A_. Thus we have _A_ and _B_ alternately filling and emptying as long as the gas burner is open. To have a continuous flow of gas in the house-pipes two pipes and two chambers are necessary, one being filled while the other is being emptied.

Fig. 52 represents the dials upon a gas meter showing a reading of 54,600 cu. ft.

[Ill.u.s.tration: FIG. 52.--Dials of a gas meter.]

=70. Centrifugal Pumps.= Fluids, such as water and air, are often put in motion by devices called _centrifugal pumps_ (see Art. 78). These pumps contain a revolving part, like a wheel without a rim, whose spokes are replaced by thin blades. This revolving part resembles the paddle wheel of some steam boats and is enclosed in a case or cover having one opening at the rim and another opening on one side about the axle.

[Ill.u.s.tration: FIG. 53.--A vacuum sweeper. (_Courtesy of the Hoover Suction Sweeper Co._)]

When the wheel is rapidly revolved, the fluid is driven out with considerable force through the opening at the rim, while a partial vacuum is produced at the axle causing a rapid flow into the device at this point.

This is the principle of the action of the _vacuum cleaner_. Fig. 53 is a section of a vacuum sweeper showing the revolving wheel and the current of air pa.s.sing into the wheel at the lower side and out of the rim of the case at the rear.

_Centrifugal water pumps_ work on the same principle and furnish a continuous flow of water, often large in volume and at considerable pressure.

Important Topics

1. Air pump.

2. Condensing pump.

3. Lift and force pumps.

4. Siphon.