The Boy's Playbook of Science Part 27

[Ill.u.s.tration: Fig. 207.--No. 1 consists of vertical permanent steel magnets and horizontal soft-iron electro-magnets which rotate.

No. 2 consists of two fixed soft-iron electro-magnets, and four bent permanent steel magnets, which rotate, in both cases of course, only when connected with the battery.]

Considering the prodigious power or _pull_ of a soft-iron electro-magnet, and its capability of supporting considerable weight, the most reasonable expectations of success might be entertained with machines acting by the direct pull. It was, however, discovered that they soon became inefficient, from the circ.u.mstance that the repeated blows received by the iron so altered its character, that it eventually a.s.sumed the quality of steel, and had a tendency to retain a certain amount of permanent magnetism, and thus to interfere with the principle of making and unmaking a magnet. It was this fact that induced Professor Jacobi, of St. Petersburg, after a large expenditure of money, to abandon arrangements of this kind, and to employ such as would at once produce a rotatory motion. The engine thus arranged was tried upon a tolerably large scale on the Neva, and by it a boat containing ten or twelve people was propelled at the rate of three miles an hour.

Various engines have been constructed by Watkins, Botta, Jacobi, Armstrong, Page, Hjorth; the engine made by the latter (Hjorth) excited much attention in 1851-52, and consisted of an electro-magnetic piston drawn within or repelled from an electro-magnetic cylinder; and by this motion it was thought that a much greater length of stroke could be secured than by the revolving wheels or discs, but the loss of power (not only in this engine, but in others) through s.p.a.ce is very great, and the lifting power of any magnet is greatly reduced and [Page 217]

altered at the smallest possible distance from its poles. This loss of power is therefore a great obstacle in the way of the useful application of electro-magnetic force, and can be appreciated even with the little models, all of which may be stopped with the slightest friction, although they may be moving at the time with great velocity.

In the second place, supposing the reduced force exerted by the two magnets, a few lines apart, was considered available for driving machinery, the moment the magnets begin to move in front of one another there is again a great loss of power, and as the speed increases, there is curiously a corresponding diminution of available mechanical power, a falling-off in the _duty_ of the engine as the rotations become more rapid. In the third place, the cost of the voltaic battery, as compared with the consumption of coal in the steam-engine, is very startling, and extremely unfavourable to electro-magnetic engines.

Mr. J. P. Joule found that the economical duty of an electro-magnetic engine at a given velocity and for a given resistance of the battery is proportioned to the mean intensity of the several pairs of the battery.

With his apparatus, every pound of zinc consumed in a Grove's battery produced a mechanical force (friction included) equal to raise a weight of 331,400 pounds to the height of one foot, when the revolving magnets were moving at the velocity of eight feet per second. Now, the _duty_ of the best Cornish steam-engine is about one million five hundred thousand pounds raised to the height of one foot by the combustion of each pound of coal, or nearly five times the extreme _duty_ that could be obtained from an electro-magnetic engine by the consumption of one pound of zinc.

This comparison is therefore so very unfavourable, that the idea of a successful application of electricity as an _economic_ source of power, is almost, if not entirely abandoned.

By inst.i.tuting a comparison between the different means of producing power, it has been shown that for every shilling expended there might be raised by

Pounds.

Manual power 600,000 one foot high in a day.

Horse 3,600,000 " "

Steam 56,000,000 " "

Electro-magnetism 900,000 " "

A powerful magnet has been compared to a steam-engine with an enormous piston but with an exceedingly short stroke. Although motive power cannot be produced from electricity and applied successfully to commercial purposes, like the steam-engine, yet the achievements of the electric telegraph as an application of a small motive power must not be lost sight of, whilst the fall of the ball at Deal and other places, by which the chronometers of the mercantile navy are regulated, as also the means of regulating the time at the General Post Office and various railway stations, are all useful applications of the power which fails to compete in other ways with steam.

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CHAPTER XVII.

THE ELECTRIC TELEGRAPH.

The engineering and philosophical details of this important instrument have grown to such formidable dimensions, that any attempt (short of devoting the whole of these pages to the subject) to give a full account of the history and application of the instrument, the failures and successes of novel inventions, and the continued onward progress of this mode of communication, must be regarded as simply impossible, and therefore a very brief account of the _principle_ only will be attempted in these pages.

For the complete history of the discovery and introduction of the principle of the Electric Telegraph the reader is referred to the Society of Arts Journal (Nos. 348-9, vol. viii.), where it is stated that it is _half a century_, dating from August, 1859, since the first galvanic telegraph was made. "It was the Russian Baron Schilling's electro-magnetic telegraph which, without its being known to be his, was brought to London, and caused the establishment of the first practically useful telegraph lines, not only in Great Britain, but in the world."

Dr. Hamel says: "The small sprout nursed on the Neva, which had been exhibited on the Rhine, and thence brought to the Thames, grew up here to a mighty tree, the fruit-laden branches of which, along with those from trees grown up since, extend more and more over the lands and seas of the Eastern hemisphere, whilst kindred trees planted in the Western hemisphere have covered that part of the world with their branches, some of which will, ere long, be interwoven with those in our hemisphere."

The first telegraph line in England was constructed by Mr. Cooke from Paddington along the Great Western Railroad to West Drayton in 1838-39; and it must be remembered that it was in February, 1837, that Mr. Cooke first consulted Professor Charles Wheatstone, having previously visited Dr. Faraday and Dr. Roget, and on the 19th November, 1837, a partnership contract was concluded between Messrs. Cooke and Wheatstone.

To the distinguished philosopher, Professor Wheatstone, the merit of the ingenious construction of the vertical-needle telegraph is due; whilst Mr. Cooke's name will always be a.s.sociated with the practical establishment of the first telegraph lines in England. The first line in the United States, from Washington to Baltimore, was completed in 1844, being arranged and worked by Professor Morse.

In British India, in April and May, 1839, the first long line of telegraph, twenty-one miles in length, and embracing 7000 feet of river surface, was constructed by Dr. (now Sir William) O'Shaughnessy.

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The construction of the electric telegraph may be considered under three heads:

1st. The Battery, _the motive power_.

2nd. The Wires, _the carriers of the force_.

3rd. The Instruments to be worked--_the bell_ and the _needle telegraph_.

THE BATTERY.

The construction and rationale of the batteries generally in use have been explained in another part of this work; those used for telegraphic purposes consist of one or more couples, of which zinc is one, the second being copper, silver, platinum, or carbon. Each couple is termed an element, and a series of such couples a _battery_.

The batteries employed chiefly on the English lines consist of a plate of cast-zinc four inches square and 3/16ths of an inch thick, attached by a copper strap one inch broad to a thin copper plate four inches square. The zinc is well amalgamated with mercury. Twelve of these couples are arranged in a trough of wood, porcelain, or gutta-percha, divided by part.i.tions into twelve water-tight cells, 1 inch wide.

The zinc and copper preserve the same order and direction throughout, and when arranged, the trough is filled with the finest white sand, and then moistened with water previously mixed with five per cent. by measure of pure sulphuric acid. This mode of applying the acid is the clever practical improvement of Mr. Cooke, and prevents any inconvenience from the spilling of the acid, and at the same time renders the battery quite portable. The voltaic arrangement thus prepared is found to remain in action for several weeks, or even months, with the occasional addition of small quant.i.ties of acid, and answers well for working needle telegraphs in fine and dry weather. In fogs and rains, at distances exceeding 200 miles at most, their action is not so perfect, and a vast number of couples must be employed, 144 to 288 being frequently in use. In France, Prussia, and America, sand batteries do not appear to answer, and Daniell's arrangement is preferred. Sixty couples suffice in France for some of the long lines--viz., from Paris to Bordeaux, 284 miles; Paris to Brussels, 231 miles; and in fact, the advantages of the Daniell's battery have become so apparent, that they are now being used on English lines. In Prussia, Bunsen's carbon battery is much used; in India, a modification of Grove's battery is preferred, the zinc being acted upon by a solution of common salt in water. Two of these elements were found sufficient to work a line of forty miles totally uninsulated, and including the sub-aqueous crossing of the Hooghly River, 6200 feet wide.

The continual energy of the battery, whatever may be its construction, depends on the circulation of the electricity, the object being to pa.s.s the force from the positive end of the series through the wires, back again to the negative extremity of the voltaic series.

The wire (the carrier of the force) must be continuous throughout, unless, of course, water or earth forms a part of the endless conducting chain.

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THE CONDUCTING WIRES.

These roads for the electricity may be of any convenient metal, and the one preferred and used is iron, which is well calculated from its great tenacity (being the most tenacious metal known) and cheapness to convey the electricity, although it is not such a good conductor as copper, and offers about six times more resistance to the flow of the current than the latter metal. The wire does not appear to be made of iron, because it is galvanized or pa.s.sed through melted zinc, which coats the surface and defends it from destructive rust, at the same time does not destroy its valuable property of tenacity or power of resisting a strain. About one ton of wire is required for every five miles, and to support this weight, stout posts of fir or larch are erected about fifty yards apart, and from ten to twenty-five feet high. At every quarter mile, on many lines, are straining-posts with ratchet wheel winders, for tightening the wires. On some of the lines the wires are attached to the posts by side brackets carrying the insulators invented by Mr. C. V. Walker, which are composed of brown salt-glazed stoneware of the hour-gla.s.s shape, as shown in the drawing. (Fig. 208.)

[Ill.u.s.tration: Fig. 208. Walker's insulator.]

There are some objections to the hour-gla.s.s insulators, and they have been modified by Mr. Edwin Clark, [Page 221] who employs a very strong stone-ware hook open at the side, so that the wire can be placed on the hook without threading, and the hooks can be replaced in case of breaking, without cutting the telegraph wire, which is securely fastened to each insulator by turns of thinner wire. An inverted cap of zinc is used to keep the insulator dry. (Fig. 209.)

[Ill.u.s.tration: Fig. 209. Clark's insulator.]

In India the conductor is rather a rod than a wire, and weighs about half a ton per mile; it is erected in the most substantial manner, and many miles of the rod are supported on granite columns, other portions on posts of the iron-wood of Arracan, or of teak.

The number of wires required by the electric telegraph often puzzles the railway traveller, and people ask why so many wires are used on some lines and so few on others? The answer is very simple: they are for convenience. Two wires only are required for the double needle telegraph, and one for the single needle instrument. But as so many instruments are required at the terminal stations, an increased number of wires, like rails for locomotives, must be provided; thus, on the Eastern Counties, seven wires are visible, and are thus employed. The two upper wires pa.s.s direct from London to Norwich; the next pair connect London, Broxbourne, Cambridge, Brandon, Chesterfield, Ely; the third pair all the small stations between London and Brandon; and the seventh wire is entirely devoted to the bell.

If the earth was not a conductor of electricity, and employed in the telegraphic circuit, four wires would be required for the double needle telegraph, and two for the single instrument. To understand this, let us suppose a battery circuit extending from Paddington to the instrument at Slough, and the wire returning from Slough to Paddington, it is evident that one wire would take the electricity to Slough, and the other return it to London, as in the diagram below. (Fig. 210.)

[Ill.u.s.tration: Fig. 210. A. The battery. B. The instrument. The arrows show the pa.s.sage of the electricity to the single needle telegraph instrument by one wire, and the return current by the other.]

If the whole of the return wire is cut away except a few feet at each end, which are connected by plates of copper with the damp earth, the current not only pa.s.ses as before, but actually has increased in intensity, and will cause a much more energetic movement of the needle in the telegraph instrument. (Fig. 211.) These plates are called "_Earth Plates_;" and Steinheil, in 1837, was the first who proved that the earth might perform the function of a wire.

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[Ill.u.s.tration: Fig. 211. A. The battery. B. The instrument. C. Earth plate at Slough. D. Earth plate at London. The arrows show the direction of the electric current.]

It must be obvious that a message may be received at any station without a battery, but in order to be able to return an answer, every station must have its own battery.

Ingeniously-constructed lightning-conductors are attached to the posts which carry the wires, so that in case of a storm, the natural electricity is conveyed to the earth, whilst the voltaic electricity artificially produced pursues its own course without deviation.

Protectors are also required for the instruments at the stations, and the plan devised by Mr. Highton is thus described by the inventor:--