The Boy's Playbook of Science Part 28

"A portion of the wire circuit--say for six or eight inches--is enveloped in blotting-paper or silk, and a ma.s.s of metallic filings, in connexion with the earth, is made to surround it. This arrangement is placed on each side of the telegraph instrument at a station. When a flash of lightning happens to be intercepted by the wires of the telegraph, the myriads of infinitesimally fine points of metal in the filings surrounding the wire at the station, on having connexion with the earth, at once draw off nearly the whole charge of lightning, and carry it safely to the earth."

THE INSTRUMENTS TO BE WORKED--THE BELL AND THE TELEGRAPH.

The bell or alarum resembles in construction that of an ordinary clock, and is in fact a piece of clockwork wound up and ready to ring a bell, when the _detent_ or preventive is removed. The detent is connected with a piece of soft iron placed before an electro-magnet, and directly the current pa.s.ses, the electro-magnet attracts the soft piece of iron attached to a perpendicular lever which the bell-crank lever rests upon; the detent is removed, and the bell rings, and again stops when the current of electricity ceases to pa.s.s.

One of the most simple alarum clocks is a common American clock, wound up daily. A small electro-magnet surrounded with thick wire is placed below a moveable piece of tinned iron, so that when this is attracted, the fly of the clock is released, and its bell tolls unceasingly while [Page 223] the magnet is excited. This arrangement is employed by Sir W.

O'Shaughnessy in the Indian telegraph system. (Fig. 212.)

It will readily be comprehended from this description that the alarum is sounded by ordinary mechanism, and that the duty of the current of the electricity is simply comprised in the act of removing the lever and liberating machinery, which may be large or small; and if it were thought necessary, the bells of the great clock-tower of the Houses of Parliament, which chime the quarters, or even "Big Ben" himself (when his const.i.tution is restored), could be rung by a person at York or Edinburgh, supposing wires, batteries, and a powerful electro-magnet with a detent mechanism for the bells, were properly arranged and connected with the clockwork.

[Ill.u.s.tration: Fig. 212. A. The soft iron tinned, which is attracted to the electro-magnet B, and liberates the detent.]

In certain cases, Mr. Charles V. Walker states that a single and distinct wire is used for the bell only, with his special mechanism, called the _ringing key_. If the bell was always on the same wire as the needle-coil, the bell would not only call the attention of, but seriously annoy the clerk (unless, of course, he happened to be a very deaf person) by its ringing whilst he was reading the signals of the needle. The nuisance is prevented by what is termed _joining over_ or making the _short circuit_--in fact, by providing for the current a shorter and much more capacious road to the needle coil than by going through that of the bell-magnet, which is made with very fine wire; and the control of the short circuit is put in the hands of the clerk.

COOKE AND WHEATSTONE'S DOUBLE NEEDLE TELEGRAPH.

The principle of this instrument, as already explained, is involved in the elementary experiment of Oersted--viz., the deflection of a magnetic needle from the inside of a coil of wire conveying a current of electricity, and as it is difficult to give a good description and drawing of the interior of the instrument that can really be understood, it may be sufficient to state that the handles give the operator the power of reversing the current of electricity, so that the needles are deflected with the utmost certainty to [Page 224] one side or the other, either separately or simultaneously. (Fig. 213.)

[Ill.u.s.tration: Fig. 213. The letters of the alphabet, figures, and a variety of conventional signals, are indicated by the single and combined movements of the needles on the dial. The left-hand needle moving once to the left indicates the +, which is given at the end of a word. Twice in the same way, A; thrice, B; first right, then left, C; the reverse, D. Once direct to the right, E; twice, F; thrice, G. In the same order with the other needle for H, I, K, L, M, N, O, P. The signals below the centre of the dial are indicated by the parallel movements of both needles simultaneously. Both needles moving once to the left indicate R; twice, S; thrice, T. First right, then left with both, U; the reverse, V. Both moving once to the right, W; twice, X; thrice, Y.

The figures are indicated in the same way as the letters nearest to which they are respectively placed. To change from letters to figures the operator gives H, followed by the +, which the recipient returns to signify that he understands. If, after the above signs (H and +) were given, C R H L were received, 1845 would be understood. A change from figures to letters is notified by giving I, followed by the +, which the recipient also returns. Each word is acknowledged. If the recipient understand, he gives E; if not, the +, in which case the word is repeated. Attention to a communication by this instrument is called by the ringing of a bell (of any size), which is effected through the agency of an electric current. The upper case contains the bell.]

Sir W. O'Shaughnessy, in his excellent work on the electric telegraph in British India, gives a description of a telegraphic instrument of remarkable simplicity, which is successfully employed in India, and is [Page 225] highly spoken of by Mr. E. V. Walker and other gentlemen practically acquainted with the working of telegraphs. It consists of a coil of fine wire on a card or ivory frame, a magnetic needle with a light index of paper pasted across it; two stops of thin sheet lead to limit the vibrations of the index; a supporting board eight inches square, and a square of gla.s.s in a frame of wood, or a common gla.s.s tumbler placed over it as a shade, to prevent the index being moved by currents of air. It is stated that the office boys, with the a.s.sistance of a native Indian carpenter, make up these telegraphs at a price not exceeding two shillings each.

In England of course they would be more expensive; but the simplicity and perfection of the arrangement are so much to be commended that we give the details for the benefit of those boys who might wish to establish a telegraph on a small scale for amus.e.m.e.nt.

THE FRAME.

This is a piece of mahogany eight inches square and one inch thick, with a hollow groove cut in its centre two inches and a half long, half an inch wide, and a quarter of an inch deep; a ledge of the same wood one inch wide and half an inch deep surrounds the frame, leaving the inner surface seven inches square; this is stained black with ink to make the motions of the index more conspicuous.

THE COIL.

This consists of fifty feet of the finest silk-covered copper wire wound on a frame of card two inches long, half an inch broad, three-eighths deep in the open part.

An edge or f.l.a.n.g.e of card, three-eighths of an inch wide, is attached to it at each side to keep the wire in its place. The frame may be of thin wood or ivory, and the winding of the wire commences at the lower left corner, and it is coiled from left to right, as the hands of a watch would move in the same plane. (Fig. 214.)

[Ill.u.s.tration: Fig. 214. The coil.]

Two inches of each end of the coil wire are now stripped of their silk covering by being rubbed with sand-paper. The coil is mounted in the frame by inserting its lower edge or f.l.a.n.g.e in the groove, so that the lower part or floor of the inside of the coil is level with that of the [Page 226] frame, as shown below, and it is now ready to receive the magnetized needle. (Fig. 215.)

[Ill.u.s.tration: Fig. 215. The coil fitted into frame.]

THE NEEDLE.

This is one inch long, one-twelfth of an inch wide, of the thinnest steel, and fitted with a little bra.s.s cap turned to a true cone to receive the point on which it is balanced. These needles are of hard tempered steel, and are magnetized by a single contact with the poles of an electro-magnet or other ordinary powerful magnet.

The magnet is now to be balanced on a steel point one-eighth of an inch high; these are nipped off with cutting pliers from common sewing needles, and soldered into a slip of thin copper three inches long, half an inch wide. (Fig. 216.)

[Ill.u.s.tration: Fig. 216. A. The needle. B. The point on the slip of copper.]

As the north end of the needle will be found to dip, it is advisable to counteract this by touching the south end with a little sh.e.l.l-lac varnish, which dries rapidly, and soon restores the needle to a perfect equilibrium.

The needle is completed for use by fixing to it an index of paper (cut from glazed letter paper) two inches long, tapering from one-eighth of an inch to a point, and fastened at right angles on to the needle with lac varnish, so as to be truly balanced, and pointing the sharp end to the east, when the needle placed on the point settles due north and south, its north pole being opposite the observer's right hand, the observer facing west. (Fig. 217.)

[Ill.u.s.tration: Fig. 217. The needle with the paper index.]

[Page 227]

The coil frame is placed north and south, and the needle is now introduced by sliding the end of the slip of copper into the opening in the frame.

To limit the vibrations of the paper index a _stop_ is placed at each side. The stops are made of a strip of thin sheet-lead or copper, a quarter of an inch broad, one inch and a half long, and turned up at a right angle, so that one inch rests on the board and half an inch is vertical. For ordinary practice these stops are placed each at half an inch from the index.

The telegraph is placed in a box, which may have a piece of looking-gla.s.s in the lid, so that the readings can be taken with the needle in the vertical instead of the horizontal position, if required.

(Fig. 218.)

[Ill.u.s.tration: Fig. 218. Box containing the telegraph, with the looking-gla.s.s in the lid. A small steel magnet is placed on or near the frame, if required, the south pole of this magnet being opposite to the north pole of the needle in the telegraph coil. The bar is four inches long, half an inch broad, three-sixteenths of an inch thick, and it is only used to counteract any local deviation which may arise in using the instrument with miles of wire. It would not be required under ordinary circ.u.mstances. The alphabet used is shown to the left.]

The ends of the fine wire of the telegraph coil are joined on to the wires from the _reversing_ instrument, and this is connected with a voltaic series of one or more elements, so that by the employment of the reverser the needle is caused to move right or left at pleasure. The [Page 228] white paper index on the black ground can be followed with the greatest certainty, and Sir W. O'Shaughnessy states that with this instrument a telegraph clerk may read at the rate of twenty words per minute with a double needle wire, being equal to forty words per minute.

THE REVERSER

consists of a block of wood, two inches and a half square, in which four hollows, half an inch deep, are cut, and these hollows are joined diagonally by copper wires let into the substance of the wood, and most carefully insulated from each other by melted cement, but exposing a clean metallic surface in each cell, which is filled with mercury. (Fig.

219.)

[Ill.u.s.tration: Fig. 219. Block of wood with four holes; the positive terminal is connected with the holes A and B, the negative with C and D; the hollows are filled with mercury. T T are the wires from the telegraph box, and it is obvious that by dipping them alternately into C B and A D the current is reversed, and the needle deflected right or left at pleasure.]

In practice a more elaborate reverser is employed, but to demonstrate the principle the simple block above described is quite sufficient.

With the telegraph placed at the top of a house, or in a distant cottage, and a single cell of Grove's battery, or at most two, for any short distances, with the reverser, messages may be pa.s.sed with great rapidity from the bottom of the house to the top, or from a mansion to the lodge, it being understood that a battery, reverser, and telegraph, are required at both places where messages are received and _answered_; but if no answers are required, the battery and reverser are placed at one end of the wire in the house, and the telegraph at the other extremity in the cottage, and earth plates may be arranged to return the current, or another wire used for that purpose.

Whilst lauding to the utmost the invention of the electric telegraph, we must remember "there is nothing new under the sun," and that after all Nature claims the _principle_ of telegraphing, and with the silent gesture, the speaking eye, interpreted and answered by others, she proclaims herself to be the originator of communication by signs.

Whilst [Page 229] the language of flowers, and the mournful requirements of the deaf and dumb in the use of the finger alphabet, show how readily man has adopted the important principle, till he has brought it to the highest state of perfection in the electric telegraph.

When the telegraph was first adopted on the Great Western Railway, the most ridiculous ideas were formed of its capabilities, and many persons firmly believed that the wires were used for the purpose of dragging letters and different articles from station to station. "Wife," said a man, looking at the telegraph wires, "I don't see, for my part, how they send letters on them wires, without tearin' 'em all to bits." "Oh, you stupid!" exclaimed his intellectual spouse; "why, they don't send the paper: they just send the writin' in a _fluid_ state."

[Ill.u.s.tration: Fig. 220. One of the ideas of telegraphic communication.]

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CHAPTER XVIII.