How it Works Part 20

[Ill.u.s.tration: FIG. 151_b._]

[Ill.u.s.tration: FIG. 151_c._--Section of a gramophone reproducer.]

In Fig. 151_c_ the construction of the gramophone reproducer is shown in section. A is the cover which screws on to the bottom B, and confines the diaphragm D between itself and a rubber ring. The portion B is elongated into a tubular shape for connection with the horn, an arm of which slides over the tube and presses against the rubber ring C to make an air-tight joint. The needle-carrier N is attached at its upper end to the centre of the diaphragm. At a point indicated by the white dot a pin pa.s.ses through it and the cover. The lower end is tubular to accommodate the steel points, which have to be replaced after pa.s.sing once over a record. A screw, S, working in a socket projecting from the carrier, holds the point fast. The record moves horizontally under the point in a plane perpendicular to the page. The groove being zigzag, the needle vibrates right and left, and rotating the carrier a minute fraction of an inch on the pivot, shakes the gla.s.s diaphragm and sends waves of air into the horn.

The gramophone is a reproducing instrument only. The records are made on a special machine, fitted with a device for causing the recorder point to describe a spiral course from the circ.u.mference to the centre of the record disc. Some gramophone records have as many as 250 turns to the inch. The total length of the tracing on a ten-inch "concert" record is about 1,000 feet.

THE MAKING OF RECORDS.

For commercial purposes it would not pay to make every record separately in a recording machine. The expense of employing good singers and instrumentalists renders such a method impracticable. All the records we buy are made from moulds, the preparation of which we will now briefly describe.

CYLINDER, OR PHONOGRAPH RECORDS.

First of all, a wax record is made in the ordinary way on a recording machine. After being tested and approved, it is hung vertically and centrally from a rotating table pivoted on a vertical metal spike pa.s.sing up through the record. On one side of the table is a piece of iron. On each side of the record, and a small distance away, rises a bra.s.s rod enclosed in a gla.s.s tube. The top of the rods are hooked, so that pieces of gold leaf may be suspended from them. A bell-gla.s.s is now placed over the record, table, and rods, and the air is sucked out by a pump. As soon as a good vacuum has been obtained, the current from the secondary circuit of an induction coil is sent into the rods supporting the gold leaves, which are volatilized by the current jumping from one to the other. A magnet, whirled outside the bell-gla.s.s, draws round the iron armature on the pivoted table, and consequently revolves the record, on the surface of which a very thin coating of gold is deposited. The record is next placed in an electroplating bath until a copper sh.e.l.l one-sixteenth of an inch thick has formed all over the outside. This is trued up on a lathe and encased in a bra.s.s tube. The "master," or original wax record, is removed by cooling it till it contracts sufficiently to fall out of the copper mould, on the inside surface of which are reproduced, in relief, the indentations of the wax "master."

Copies are made from the mould by immersing it in a tank of melted wax.

The cold metal chills the wax that touches it, so that the mould soon has a thick waxen lining. The mould and copy are removed from the tank and mounted on a lathe, which shapes and smooths the inside of the record. The record is loosened from the mould by cooling. After inspection for flaws, it is, if found satisfactory, packed in cotton-wool and added to the saleable stock.

Gramophone master records are made on a circular disc of zinc, coated over with a very thin film of acid-proof fat. When the disc is revolved in the recording machine, the sharp stylus cuts through the fat and exposes the zinc beneath. On immersion in a bath of chromic acid the bared surfaces are bitten into, while the unexposed parts remain unaffected. When the etching is considered complete, the plate is carefully cleaned and tested. A negative copper copy is made from it by electrotyping. This const.i.tutes the mould. From it as many as 1,000 copies may be made on ebonite plates by combined pressure and heating.

[32] The Edison Bell phonograph is here referred to.

[33] Some of the sibilant or hissing sounds of the voice are computed to be represented by depressions less than a millionth of an inch in depth.

Yet these are reproduced very clearly!

Chapter XVII.

WHY THE WIND BLOWS.

Why the wind blows--Land and sea breezes--Light air and moisture--The barometer--The column barometer--The wheel barometer--A very simple barometer--The aneroid barometer--Barometers and weather--The diving-bell--The diving-dress--Air-pumps--Pneumatic tyres--The air-gun--The self-closing door-stop--The action of wind on oblique surfaces--The balloon--The flying-machine.

When a child's rubber ball gets slack through a slight leakage of air, and loses some of its bounce, it is a common practice to hold it for a few minutes in front of the fire till it becomes temporarily taut again.

Why does the heat have this effect on the ball? No more air has been forced into the ball. After perusing the chapter on the steam-engine the reader will be able to supply the answer. "Because the molecules of air dash about more vigorously among one another when the air is heated, and by striking the inside of the ball with greater force put it in a state of greater tension."

If we heat an open jar there is no pressure developed, since the air simply expands and flows out of the neck. But the air that remains in the jar, being less in quant.i.ty than when it was not yet heated, weighs less, though occupying the same s.p.a.ce as before. If we took a very thin bladder and filled it with hot air it would therefore float in colder air, proving that heated air, as we should expect, _tends to rise_. The fire-balloon employs this principle, the air inside the bag being kept artificially warm by a fire burning in some vessel attached below the open neck of the bag.

Now, the sun s.h.i.+nes with different degrees of heating power at different parts of the world. Where its effect is greatest the air there is hottest. We will suppose, for the sake of argument, that, at a certain moment, the air envelope all round the globe is of equal temperature.

Suddenly the sun s.h.i.+nes out and heats the air at a point, A, till it is many degrees warmer than the surrounding air. The heated air expands, rises, and spreads out above the cold air. But, as a given depth of warm air has less weight than an equal depth of cold air, the cold air at once begins to rush towards B and squeeze the rest of the warm air out.

We may therefore picture the atmosphere as made up of a number of colder currents pa.s.sing along the surface of the earth to replace warm currents rising and spreading over the upper surface of the cold air. A similar circulation takes place in a vessel of heated water (see p. 17).

LAND AND SEA BREEZES.

A breeze which blows from the sea on to the land during the day often reverses its direction during the evening. Why is this? The earth grows hot or cold more rapidly than the sea. When the sun s.h.i.+nes hotly, the land warms quickly and heats the air over it, which becomes light, and is displaced by the cooler air over the sea. When the sun sets, the earth and the air over it lose their warmth quickly, while the sea remains at practically the same temperature as before. So the balance is changed, the heavier air now lying over the land. It therefore flows seawards, and drives out the warmer air there.

LIGHT AIR AND MOISTURE.

Light, warm air absorbs moisture. As it cools, the moisture in it condenses. Breathe on a plate, and you notice that a watery film forms on it at once. The cold surface condenses the water suspended in the warm breath. If you wish to dry a damp room you heat it. Moisture then pa.s.ses from the walls and objects in the room to the atmosphere.

THE BAROMETER.

This property of air is responsible for the changes in weather. Light, moisture-laden air meets cold, dry air, and the sudden cooling forces it to release its moisture, which falls as rain, or floats about as clouds.

If only we are able to detect the presence of warm air-strata above us, we ought to be in a position to foretell the weather.

We can judge of the specific gravity of the air in our neighbourhood by means of the barometer, which means "weight-measurer." The normal air-pressure at sea-level on our bodies or any other objects is about 15 lbs. to the square inch--that is to say, if you could imprison and weigh a column of air one inch square in section and of the height of the world's atmospheric envelope, the scale would register 15 lbs. Many years ago (1643) Torricelli, a pupil of Galileo, first calculated the pressure by a very simple experiment. He took a long gla.s.s tube sealed at one end, filled it with mercury, and, closing the open end with the thumb, inverted the tube and plunged the open end below the surface of a tank of mercury. On removing his thumb he found that the mercury sank in the tube till the surface of the mercury in the tube was about 30 inches in a vertical direction above the surface of the mercury in the tank.

Now, as the upper end was sealed, there must be a vacuum _above_ the mercury. What supported the column? The atmosphere. So it was evident that the downward pressure of the mercury exactly counterbalanced the upward pressure of the air. As a mercury column 30 inches high and 1 inch square weighs 15 lbs., the air-pressure on a square inch obviously is the same.

[Ill.u.s.tration: FIG. 152.--A Fortin barometer.]

FORTIN'S COLUMN BAROMETER

is a simple Torricellian tube, T, with the lower end submerged in a little gla.s.s tank of mercury (Fig. 152). The bottom of this tank is made of washleather. To obtain a "reading" the screw S, pressing on the washleather, is adjusted until the mercury in the tank rises to the tip of the little ivory point P. The reading is the figure of the scale on the face of the case opposite which the surface of the column stands.

[Ill.u.s.tration: FIG. 153.]

THE WHEEL BAROMETER

also employs the mercury column (Fig. 153). The lower end of the tube is turned up and expanded to form a tank, C. The pointer P, which travels round a graduated dial, is mounted on a spindle carrying a pulley, over which pa.s.ses a string with a weight at each end. The heavier of the weights rests on the top of the mercury. When the atmospheric pressure falls, the mercury in C rises, lifting this weight, and the pointer moves. This form of barometer is not so delicate or reliable as Fortin's, or as the siphon barometer, which has a tube of the same shape as the wheel instrument, but of the same diameter from end to end except for a contraction at the bend. The reading of a siphon is the distance between the two surfaces of the mercury.

A VERY SIMPLE BAROMETER

is made by knocking off the neck of a small bottle, filling the body with water, and hanging it up by a string in the position shown (Fig.

154). When the atmospheric pressure falls, the water at the orifice bulges outwards; when it rises, the water retreats till its surface is slightly concave.

[Ill.u.s.tration: FIG. 154.]

THE ANEROID BAROMETER.

On account of their size and weight, and the comparative difficulty of transporting them without derangement of the mercury column, column barometers are not so generally used as the aneroid variety. Aneroid means "without moisture," and in this particular connection signifies that no liquid is used in the construction of the barometer.

Fig. 155 shows an aneroid in detail. The most noticeable feature is the vacuum chamber, V C, a circular box which has a top and bottom of corrugated but thin and elastic metal. Sections of the box are shown in Figs. 156, 157. It is attached at the bottom to the base board of the instrument by a screw (Fig. 156). From the top rises a pin, P, with a transverse hole through it to accommodate the pin K E, which has a triangular section, and stands on one edge.

[Ill.u.s.tration: FIG. 155.--An aneroid barometer.]

Returning to Fig. 155, we see that P projects through S, a powerful spring of sheet-steel. To this is attached a long arm, C, the free end of which moves a link rotating, through the pin E, a spindle mounted in a frame, D. The spindle moves arm F. This pulls on a very minute chain wound round the pointer spindle B, in opposition to a hairspring, H S. B is mounted on arm H, which is quite independent of the rest of the aneroid.

[Ill.u.s.tration: FIG. 156. FIG. 157. The vacuum chamber of an aneroid barometer extended and compressed.]