WONDERS OF SCIENCE.

THE ELECTRIC TELEGRAPH.

THE application of electricity to the conveyance of messages is now so general, not only in our delightful little island, but on the Continent, that we are anxious all alike should know something of the principles and means whereby this is effected. To those who are already well read in the subjects of electricity and magnetism, this knowledge may be easily acquired, there being many works already published which treat the matter in a very scientific and

complete manner; but there is the same objection to all, namely, that they are not suited to the capacities of the young. The chief feature of the FAMILY FRIEND has always been, that it has supplied this deficiency in all the subjects of which it treats. There science loses its difficulties, and instruction is imparted by able writers in such a familiar manner, that learning becomes, even to the child, an amusement and a pleasure. In the description of the electric telegraph, therefore, we will follow the good example thus set us, and, by laying aside all technical and scientific terms,

explain clearly this greatest wonder of our day. The source of the electricity used requires our first attention. This is what is called a voltaic, or galvanic battery; and it is so called from Volta and Galvani, its originators. We can make a very simple battery by means of two tumblers, a little salt and water, two small pieces of zinc, and two of copper, united in the following manner :-A and B are the tumblers, c c the pieces of copper, z z the pieces of zinc; the tumblers being partly filled with the salt and water, the battery is complete.

You will observe that the metals used are dissimilar; that a plate of copper and one of zinc unite at e, and that there are wires fixed to the other two plates, which, as yet, are in no way connected. Whilst things are in this state, nothing will take place; the battery is at rest, and no electricity is evolved by it; but if we join the two wires, a current of electricity will immediately pass, and this current will continue till we again separate the wires. You will naturally ask the reason of this; but we may tell you that it is not known. We might enter into what some may call an explanation of it; but after puzzling you with all kinds of technicalities, and scientific terms, you would be none the wiser,.and we should but return to where we began. Yet, if we cannot explain the cause, we can give you a general idea of the effect; and point out this simple rule-If two plates of metal are placed in a solution which will only dissolve one of them, and their upper edges are brought into contact, whilst the others are kept apart, a current will pass from one to the other through the solution, and passing also from one to the other at the point of contact, will continue thus circulating, till either the soluble matter is consumed, or the liquid itself is saturated that is, has dissolved as much metal as it is capable of dissolving. This is always

the case; but often the effect is so slight, that it is rarely perceptible. Take a piece of silver, and a piece of zinc the size of a half-crown; place one upon the tip of the tongue, the other under it; bring their edges into contact, and what is called a shock will be perceptible; that is, the saliva acting upon the zinc and not upon the silver, a small battery is made, and the electricity passes from the zinc through the tongue to the silver, thence to the zinc again, and thus circulates till you part the edges of the metals. The shock is very slight, being chiefly known by an acid taste; nor would it be felt at all, but that the tongue is so acutely sensitive. We have called this a small battery, but it is scarcely a correct term; it is a single voltaic paira battery, in its proper sense, being made up by a union of two or more such pairs, as in the case of the one above. In practice, a battery consists of twelve or more such pairs; and the following sketch represents one commonly used in working the electric telegraph:

Fig. 2.

•n

Fig. 3.

It contains twenty-four pairs of zinc and copper plates, about four inches square. Each pair are soldered together by means of a strap of metal, as in Fig. 3. To make it quite clear, we have drawn but twelve pairs in section, and lettered the alternate plates, z standing for zinc, and c for copper. The trough in which they are placed is either made of baked wood, glass, earthen-ware, or gutta-percha; the only requisite being, that it is a non-conductor of electricitythat is, such a substance as electricity will not readily pass through. The last plate at one end is zinc, and the other copper, and a wire is soldered to each. If these wires are joined, a current of electricity will pass from the zinc plate to which one is attached, through the exciting liquid, which is here sulphuric acid and water, to the copper plate in the same cell, thence by the metal;

strap to the zinc of the next cell, and thus through the whole series to the other single plate, whence it passes by the wires to the first zinc plate again. But as each pair of plates produce similar currents, their combined power is very great, and a large quantity of electricity passes through the terminal wires. So much for the battery. We will now go a step further, and learn an extraordinary effect which it is capable of producing. You know what a magnet is, and many of you have, we dare say, seen a mariner's compass-if not, we must briefly tell you what it is. It consists of a flat piece of steel, of this shape, which is called a needle. This is suspended on a point, by means of a small hole, or rather conical indentation made in its centre. This needle being magnetised, and thus suspended, will always point in the same direction, one end being directed to the north, the other to the south; and it thus enables the sailor to go in any direction he may desire. For if he knows where these points are, he can tell the east and west; and if, as is always done, a card is placed below the needle, with the intermediate points carefully marked upon it, he has no difficulty in steering exactly to any place of which he knows the position; and thus the compass is to the sailor on the pathless deep just what the direction-post is to the traveller. Now, if we take such a needle as we have described, and suspend it vertically on an axis passing through its centre, and then, by means of a wire, pass a current from our battery round it thus,

order to attain this position, depends on the direction of the current. Thus we have arrived at the principle of an electric, telegraph. We have but to agree upon a set of signals, that the deflection of the needle shall signify; and if we can contrive to send the current in the direction we wish,' so as to move the north pole of the needle to the right or left at will, the apparatus will be complete. But, in practice, it is necessary that we should be able to move, as we please, a similar needle to our own, at the station to which we desire to send the message. In order to accomplish this, we have but to conduct the current from one station to the other by means of an insulated wire. This will be easily understood by the following diagram, where the

battery is represented at A, and the different stations at B, at each of which the needles have their north poles upwards; and the wire conveying the current passes in the same direction round all, and lastly returns to the other pole of the battery; thus the current, leaving the battery by the copper or positive end, c, will traverse the wire in the direction of the arrows, deflect the needles in the same direction at the different stations, and return to the zinc or negative end of the battery by the same wire.

We will now tell you a very curious fact about the return of the current. The return wire is not necessary, nor is it now put in practice; for a man named Sternheil proved, in 1837, that if we buried deep. in the ground the end of the wire, attaching it to a plate of metal, the earth itself would conduct the current back again, thus saving the cost of a return wire. Thus, in the figure above, we have represented this by dotted lines, and marked the direction of the return current by double arrows,

NO being the plates of metal, and having the wires attached to them.

I must now try and explain to you the instrument by which communication is made with the battery, and which is qualified not only to send, but also to receive signals. It is represented below; Fig. 7 being the exterior, and Fig. 8 the interior. The needles in Fig. 7 (for there are usually two) are on the same axis as the ones on which the electric current acts, only their poles are reversed, the north pole of the one being opposite the south pole of the other, by which the effect of the earth's magnetism is annulled, and they are the more powerfully influenced by the electric current. It is by means of these outer needles that the signals are read: they are prevented from deflecting too far from their vertical or upright position by two ivory studs, one on each side; and thus the signalling is rendered more certain and rapid than if they were allowed to oscillate further.

at the figure, and read this description of receiving signals several times, till you see it clearly; for though, at first sight, the apparatus appears very complicated, it is not so; these slips of brass, so curiously shaped, being all that is required to receive signals; to send them, the cylinder C is added, the action of which we will now explain. The furthest end of it is joined to one of the handles seen in Fig. 7, by which it is made to revolve in either direction. Supposing, then, we move this handle so as to cause the small metal pin z to press against the spring e, we can thus remove the end of this spring from the short bar against which it rested, whilst the pin y at the other end of the cylinder will touch the curved end of the slip F (both these pins are fixed into the metal caps at the ends of the cylinder). The current will now pass from the battery by the spring H to the brass cap of the cylinder, thence by the pin z to e; é being removed from the short bar, and the current thus cut off in that direction, it will pass to x, which is a part of the same slip as é, thence round the coil deflecting the needle, and passing to the next station by the slip o, and wire W, will deflect the needle there, and return by the earth current to W". Although it is a crooked path, the electric current traverses it so quickly that no perceptible time elapses between the movement of the needle at our own instrument and the various needles of all the telegraphs on the line. Each handle has a separate cylinder, and each needle a separate coil, one only being represented for the sake of clearness. Every word of the message sent is spelt letter by letter, according to the number of times that each needle moves. The following is one of the usual alphabets, and (as in Fig. 7) this is commonly inscribed on the face of the instrument. It is the code of a single needle:

A-One movement to the left.
B-Two left.

C-Three left.

O-Two right,
P-Three right.
Q-Four right.

R-One right, one left.
S-Two right, one left.
T-Three right, one left.
U-One right, two left.
V-Two right, two left.
W-Three right, two left.
X-One right, three left.
Y-Two right, three left.
Z-One right, four left.

With two needles the alphabet is somewhat different; but you will now understand how the movement of the needles can signify words; and we think you must now have a very good idea of the machinery of an electric telegraph. We shall now show you how the alarum is rung by electricity, that a message is about to be sent to him, to give the clerk at the instrument notice that he may be at his post, and ready to watch the needles, and read.

Wonderful as it may appear, an electric current passing round a piece of soft iron will instantly convert it into a magnet but its magnetic properties cease as soon as the current stops. In the telegraph alarum this effect of electricity is thus applied:-A is a piece of soft iron, bent

D

A

D-Four left.

E-One left, one right.
F-One left, two right.
G-One left. three right.
H-Two left, one right.
I-Two left, two right.
J-Two left, three right.
K-Three left, one right.
L-Three left, two right.
M-Four left, one right.
N-One right.

C

Fig. 9.

into the form of a horse-shoe; some covered copper wire is wound round it, the ends, B and C, being left loose for the purpose of connecting them with the battery. D is a piece of steel, connected with the lever, E; the other end of which forms a detent or catch, which falls into one of the notches in the wheel, F. This wheel, when the catch is removed, will revolve by a spring,

R