How it Works Part 7

Reference was made in connection with the electrical ignition of internal-combustion engines (p. 101) to the _induction coil_. This is a device for increasing the _voltage_, or pressure, of a current. The two-cell acc.u.mulator carried in a motor car gives a voltage (otherwise called electro-motive force = E.M.F.) of 44 volts. If you attach a wire to one terminal of the acc.u.mulator and brush the loose end rapidly across the other terminal, you will notice that a bright spark pa.s.ses between the wire and the terminal. In reality there are two sparks, one when they touch, and another when they separate, but they occur so closely together that the eye cannot separate the two impressions. A spark of this kind would not be sufficiently hot to ignite a charge in a motor cylinder, and a spark from the induction coil is therefore used.

[Ill.u.s.tration: FIG. 53.--Sketch of an induction coil.]

We give a sketch of the induction coil in Fig. 53. It consists of a core of soft iron wires round which is wound a layer of coa.r.s.e insulated wire, denoted by the thick line. One end of the winding of this _primary_ coil is attached to the battery, the other to the base of a hammer, H, vibrating between the end of the core and a screw, S, pa.s.sing through an upright, T, connected with the other terminal of the battery.

The action of the hammer is precisely the same as that of the armature of an electric bell. Outside the primary coil are wound many turns of a much finer wire completely insulated from the primary coil. The ends of this _secondary_ coil are attached to the objects (in the case of a motor car, the insulated wire of the sparking-plug and a wire projecting from its outer iron casing) between which a spark has to pa.s.s. As soon as H touches S the circuit is completed. The core becomes a powerful magnet with external lines of force pa.s.sing from one pole to the other over and among the turns of the secondary coil. H is almost instantaneously attracted by the core, and the break occurs. The lines of force now (at least so it is supposed) sink into the core, cutting through the turns of the "secondary," and causing a powerful current to flow through them. The greater the number of turns, the greater the number of times the lines of force are cut, and the stronger is the current. If sufficiently intense, it jumps any gap in the secondary circuit, heating the intermediate air to a state of incandescence.

THE CONDENSER.

The sudden parting of H and S would produce strong sparking across the gap between them if it were not for the condenser, which consists of a number of tinfoil sheets separated by layers of paraffined paper. All the "odd" sheets are connected with T, all the "even" with T^1. Now, the more rapid the extinction of magnetism in the core after "break" of the primary circuit, the more rapidly will the lines of force collapse, and the more intense will be the induced current in the secondary coil.

The condenser diminishes the period of extinction very greatly, while lengthening the period of magnetization after the "make" of the primary current, and so decreasing the strength of the reverse current.

TRANSFORMATION OF CURRENT.

The difference in the voltage of the primary and secondary currents depends on the length of the windings. If there are 100 turns of wire in the primary, and 100,000 turns in the secondary, the voltage will be increased 1,000 times; so that a 4-volt current is "stepped up" to 4,000 volts. In the largest induction coils the secondary winding absorbs 200-300 miles of wire, and the spark given may be anything up to four feet in length. Such a spark would pierce a gla.s.s plate two inches thick.

It must not be supposed that an induction coil increases the _amount_ of current given off by a battery. It merely increases its pressure at the expense of its volume--stores up its energy, as it were, until there is enough to do what a low-tension flow could not effect. A fair comparison would be to picture the energy of the low-tension current as the momentum of a number of small pebbles thrown in succession at a door, say 100 a minute. If you went on pelting the door for hours you might make no impression on it, but if you could knead every 100 pebbles into a single stone, and throw these stones one per minute, you would soon break the door in.

Any intermittent current can be transformed as regards its intensity.

You may either increase its pressure while decreasing its rate of flow, or _amperage_; or decrease its pressure and increase its flow. In the case that we have considered, a continuous battery current is rendered intermittent by a mechanical contrivance. But if the current comes from an "alternating" dynamo--that is, is already intermittent--the contact-breaker is not needed. There will be more to say about transformation of current in later paragraphs.

USES OF THE INDUCTION COIL.

The induction coil is used--(1.) For pa.s.sing currents through gla.s.s tubes almost exhausted of air or containing highly rarefied gases. The luminous effects of these "Geissler" tubes are very beautiful. (2.) For producing the now famous X or Rontgen rays. These rays accompany the light rays given off at the negative terminal (cathode) of a vacuum tube, and are invisible to the eye unless caught on a fluorescent screen, which reduces their rate of vibration sufficiently for the eye to be sensitive to them. The Rontgen rays have the peculiar property of penetrating many substances quite opaque to light, such as metals, stone, wood, etc., and as a consequence have proved of great use to the surgeon in localizing or determining the nature of an internal injury.

They also have a deterrent effect upon cancerous growths. (3.) In wireless telegraphy, to cause powerful electric oscillations in the ether. (4.) On motor cars, for igniting the cylinder charges. (5.) For electrical ma.s.sage of the body.

[11] "What is Electricity?" p. 46.

[12] If a magnetized bar be heated to white heat and tapped with a hammer it loses its magnetism, because the distance between the molecules has increased, and the molecules can easily return to their original positions.

Chapter VI.

THE ELECTRIC TELEGRAPH.

Needle instruments--Influence of current on the magnetic needle--Method of reversing the current--Sounding instruments--Telegraphic relays--Recording telegraphs--High-speed telegraphy.

Take a small pocket compa.s.s and wind several turns of fine insulated wire round the case, over the top and under the bottom. Now lay the compa.s.s on a table, and turn it about until the coil is on a line with the needle--in fact, covers it. Next touch the terminals of a battery with the ends of the wire. The needle at once shifts either to right or left, and remains in that position as long as the current flows. If you change the wires over, so reversing the direction of the current, the needle at once points in the other direction. It is to this conduct on the part of a magnetic needle when in a "magnetic field" that we owe the existence of the needle telegraph instrument.

NEEDLE INSTRUMENTS.

[Ill.u.s.tration: FIG. 54.--Sketch of the side elevation of a Wheatstone needle instrument.]

Probably the best-known needle instrument is the Cooke-Wheatstone, largely used in signal-boxes and in some post-offices. A vertical section of it is shown in Fig. 54. It consists of a base, B, and an upright front, A, to the back of which are attached two hollow coils on either side of a magnetic needle mounted on the same shaft as a second dial needle, N, outside the front. The wires W W are connected to the telegraph line and to the commutator, a device which, when the operator moves the handle H to right and left, keeps reversing the direction of the current. The needles on both receiving and transmitting instruments wag in accordance with the movements of the handle. One or more movements form an alphabetical letter of the Morse code. Thus, if the needle points first to left, and then to right, and comes to rest in a normal position for a moment, the letter A is signified; right-left-left-left in quick succession = B; right-left-right-left = C, and so on. Where a marking instrument is used, a dot signifies a "left,"

and a dash a right; and if a "sounder" is employed, the operator judges by the length of the intervals between the clicks.

INFLUENCE OF CURRENT ON A MAGNETIC NEEDLE.

[Ill.u.s.tration: FIGS. 55, 56.--The coils of a needle instrument. The arrows show the direction taken by the current.]

Figs. 55 and 56 are two views of the coils and magnetic needle of the Wheatstone instrument as they appear from behind. In Fig. 55 the current enters the left-hand coil from the left, and travels round and round it in a clockwise direction to the other end, whence it pa.s.ses to the other coil and away to the battery. Now, a coil through which a current pa.s.ses becomes a magnet. Its polarity depends on the direction in which the current flows. Suppose that you are looking through the coil, and that the current enters it from your end. If the wire is wound in a clockwise direction, the S. pole will be nearest you; if in an anti-clockwise direction, the N. pole. In Fig. 55 the N. poles are at the right end of the coils, the S. poles at the left end; so the N. pole of the needle is attracted to the right, and the S. pole to the left. When the current is reversed, as in Fig. 56, the needle moves over. If no current pa.s.ses, it remains vertical.

METHOD OF REVERSING THE CURRENT.

[Ill.u.s.tration: FIG. 57.--General arrangement of needle-instrument circuit. The shaded plates on the left (B and R) are in contact.]

A simple method of changing the direction of the current in a two-instrument circuit is shown diagrammatically in Fig. 57. The _principle_ is used in the Wheatstone needle instrument. The battery terminals at each station are attached to two bra.s.s plates, A B, A^1 B^1. Crossing these at right angles (under A A^1 and over B B^1) are the flat bra.s.s springs, L R, L^1 R^1, having b.u.t.tons at their lower ends, and fixed at their upper ends to baseboards. When at rest they all press upwards against the plates A and A^1 respectively. R and L^1 are connected with the line circuit, in which are the coils of dials 1 and 2, one at each station. L and R^1 are connected with the earth-plates E E^1. An operator at station 1 depresses R so as to touch B. Current now flows from the battery to B, thence through R to the line circuit, round the coils of both dials through L^1 A^1 and R to earth-plate E^1, through the earth to E, and then back to the battery through L and A. The needles a.s.sume the position shown. To reverse the current the operator allows R to rise into contact with A, and depresses L to touch B. The course can be traced out easily.

In the Wheatstone "drop-handle" instrument (Fig. 54) the commutator may be described as an insulated core on which are two short lengths of bra.s.s tubing. One of these has rubbing against it a spring connected with the + terminal of the battery; the other has similar communication with the - terminal. Projecting from each tube is a spike, and rising from the baseboard are four upright bra.s.s strips not quite touching the commutator. Those on one side lead to the line circuit, those on the other to the earth-plate. When the handle is turned one way, the spikes touch the forward line strip and the rear earth strip, and _vice versa_ when moved in the opposite direction.

SOUNDING INSTRUMENTS.

Sometimes little bra.s.s strips are attached to the dial plate of a needle instrument for the needle to strike against. As these give different notes, the operator can comprehend the message by ear alone. But the most widely used sounding instrument is the Morse sounder, named after its inventor. For this a reversible current is not needed. The receiver is merely an electro-magnet (connected with the line circuit and an earth-plate) which, when a current pa.s.ses, attracts a little iron bar attached to the middle of a pivoted lever. The free end of the lever works between two stops. Every time the circuit is closed by the transmitting key at the sending station the lever flies down against the lower stop, to rise again when the circuit is broken. The duration of its stay decides whether a "long" or "short" is meant.

TELEGRAPHIC RELAYS.

[Ill.u.s.tration: FIG. 58.--Section of a telegraph wire insulator on its arm. The shaded circle is the line wire, the two blank circles indicate the wire which ties the line wire to the insulator.]

When an electric current has travelled for a long distance through a wire its strength is much reduced on account of the resistance of the wire, and may be insufficient to cause the electro-magnet of the sounder to move the heavy lever. Instead, therefore, of the current acting directly on the sounder magnet, it is used to energize a small magnet, or _relay_, which pulls down a light bar and closes a second "local"

circuit--that is, one at the receiver end--worked by a separate battery, which has sufficient power to operate the sounder.

RECORDING TELEGRAPHS.

By attaching a small wheel to the end of a Morse-sounder lever, by arranging an ink-well for the wheel to dip into when the end falls, and by moving a paper ribbon slowly along for the wheel to press against when it rises, a self-recording Morse inker is produced. The ribbon-feeding apparatus is set in motion automatically by the current, and continues to pull the ribbon along until the message is completed.

The Hughes type-printer covers a sheet of paper with printed characters in bold Roman type. The transmitter has a keyboard, on which are marked letters, signs, and numbers; also a type-wheel, with the characters on its circ.u.mference, rotated by electricity. The receiver contains mechanisms for rotating another type-wheel synchronously--that is, in time--with the first; for shifting the wheel across the paper; for pressing the paper against the wheel; and for moving the paper when a fresh line is needed. These are too complicated to be described here in detail. By means of relays one transmitter may be made to work five hundred receivers. In London a single operator, controlling a keyboard in the central dispatching office, causes typewritten messages to spell themselves out simultaneously in machines distributed all over the metropolis.

The tape machine resembles that just described in many details. The main difference is that it prints on a continuous ribbon instead of on sheets.

Automatic electric printers of some kind or other are to be found in the vestibules of all the princ.i.p.al hotels and clubs of our large cities, and in the offices of bankers, stockbrokers, and newspaper editors. In London alone over 500 million words are printed by the receivers in a year.

HIGH-SPEED TELEGRAPHY.

At certain seasons, or when important political events are taking place, the telegraph service would become congested with news were there not some means of transmitting messages at a much greater speed than is possible by hand signalling. Fifty words a minute is about the limit speed that a good operator can maintain. By means of Wheatstone's _automatic transmitter_ the rate can be increased to 400 words per minute. Paper ribbons are punched in special machines by a number of clerks with a series of holes which by their position indicate a dot or a dash. The ribbons are pa.s.sed through a special transmitter, over little electric brushes, which make contact through the holes with surfaces connected to the line circuit. At the receiver end the message is printed by a Morse inker.