Obrázky na stránke

electricity produced in this way is called Galvanism,1 or Voltaic Electricity. When two plates, one of copper, and the other of zinc rubbed over with mercury, are placed in contact in a vessel of water in which is a little sulphuric acid, bubbles of gas are formed; and when the plates are

removed, the zinc one is found to have lost in weight, the part it has lost being dissolved in the water. The same action takes place, although the plates are not in contact, if they are joined by a copper wire, as in the figure. In consequence of this chemical action, it is found that a current of electricity passes along the wire from the copper plate, in which positive electricity is produced, to the zinc, in which negative is produced. Further, under the liquid, a current passes from the zinc plate to the copper; so that by means of the wire and the liquid, the electricity makes a complete circuit; and such an arrangement receives the name of a galvanic or voltaic circuit. By employing a number of plates, the zinc ones being connected with the copper ones in a series, the strength of the current is of course increased, and such an arrangement is called a galvanic battery.


Fig. 34.



For the proper understanding of what is to follow, it will be necessary to give here a short description of magnets. There is a certain ore of iron which has the power of attracting iron. It was first found in Magnesia or Lydia, in Asia Minor; hence a piece of this ore was called a Magnet, or Lydian-stone, which last name (probably from the power of the ore to lead or attract things) became changed into loadstone. When a small magnetic bar is nicely balanced on a fine point, it is called a magnetic needle, and has the remarkable property of always pointing north and south, being of course free to move.3 Such a piece of ore is called a natural magnet. An artificial magnet can be

Fig. 35.

1 So called from Galvani of Bologna, its discoverer.

2 Voltaic Electricity, or Voltaism, so called from Volta, an Italian.

3 The mariner's compass consists simply of a needle of this kind, balanced in the centre of a circular box, on the edge of which are letters for the different points, N., S., E., W., &c.

made by rubbing a piece of iron with a natural magnet; but the strongest magnets are got by coiling the wire of a galvanic battery round a piece of iron. While the electric current is passing through the wire, the iron becomes strongly magnetic, and ceases to be so as soon as the current is stopped. Magnets are of different forms. Perhaps the most common form is that of the horse-shoe magnet (fig. 36), the object being to bring together the two ends, called the poles, N and S, in which the strength of the magnet is concentrated. The part sn is not properly a portion of the magnet, but is a piece of iron, called an armature,1 used partly for convenience, but chiefly for keeping in the magnetism. Fig. 37 shews an electro-magnet, which is a piece of iron that can be rendered magnetic by an electric current, as described above. Neither the horse-shoe nor the armature is magnetic in itself, and therefore they will not remain in

[graphic][merged small][merged small][merged small][merged small]

contact; but as soon as the electric current is sent through the wire coiled round the magnet, the armature is pulled to it with a sharp click. We now proceed to describe the working of the Electric Telegraph.

There is one kind of telegraph which depends on the effect that an electric current has on a magnetic needle placed near it, as in fig. 34, of turning it out of its natural position, in which it points north and south; but as we can only describe one, it will be one on another principle, which is perhaps the best and the one now most extensively adopted, namely, the electro-magnetic. Of this, again, there are various modifications; instrument here described is Morse's.


We have seen that an electro-magnet is only magnetised when the current is passing, and this can only be when the circuit is complete, as in fig. 34, by the wire and the liquid. Now, everybody knows how the wire of the telegraph passes from one station to another; but how about the

1 From Latin armatura, armour, protection.

other part of the circuit-the part played by the liquid in fig. 34? In the case of the telegraph, the earth is substituted for the liquid, for if the wire which goes along the lines be attached to the copper plate of the battery, and a wire, attached to the zinc, be carried down into the earth, the circuit is complete. In fig. 38, then, L is the line-wire, and E the earth-wire, both of which are made continuous with the coils of wire on the electromagnet, MM'; the armature, A, is attached to a lever, ll', which turns on the axis k. Whenever the current is made to pass through the wire, the armature is drawn down, bringing the end of the lever with it; this raises the other end, to which is fixed a sharp point, p; opposite this point is a

[graphic][ocr errors][subsumed][subsumed][merged small]

groove in the roller, r, and over this groove is made to pass a slip of paper, PP', which is made to move towards P' by the rollers, rr'. These rollers are worked by clock-work, independently of the rest of the machine. When the point p is raised into the groove on the roller r, a raised mark is made on the upper surface of the paper, which will be a dot or a line according to the time the point is raised, that is, according to the time the circuit is kept complete; as soon as the current ceases, the armature is left by the magnet free to rise, and the end of the lever, with the point, is


Fig. 39.



[ocr errors][merged small]

pulled down by the spring s. By
means of the dots and lines thus
made on the paper, an alphabet
is constructed, and words can
thus be written down at the dis-
tance of thousands of miles.
how is this system

But now, of stopping and setting agoing of the current managed at the station from which the message is sent? It is done by what is called a key, fig. 39. This is a lever, ll, which moves on an axis, A, and is worked by

a handle, H. To the key are attached three wires: the line-wire, L; a wire, E, attached to a Morse's Recorder, at its own station; and one, C, attached to the copper plate of a galvanic battery. The ordinary position of the key is as seen in the figure, the nipple n, in contact with the little anvil b, that end of the lever being held down by the spring, 8; so that, when a message is to be received, the current passes from L, by A, l, n, b, and E, to the recorder. But when a message is to be sent to another station, the handle, H, is pressed down, the contact between n and b is broken, and the nipple m, is now brought down on the anvil a. This connects the copper plate of the battery with the line-wire to the distant station; and the current passes from the copper plate through the linewire, through the key at the distant station (which is in the position seen in fig. 39), and through the coils of the electro-magnet of the recorder. From the recorder it passes down the earth-wire, and then back, through the earth, to the zinc plate of the battery. The time during which the sharp point in the recorder is to press upon the paper, as before described, is thus regulated by means of the handle, H.


1. Development of Heat.-HEAT may be produced by mechanical means in three ways, by friction or the resistance of the surfaces of two bodies when rubbed, by percussion or the striking of one body against another, and by compression.

(1) By Friction.-If a smooth metal button be stuck on a cork, and rubbed on a piece of soft deal-wood, as a form, it will become heated by the friction; and if rubbed long enough, will become so hot as to scorch wood and paper, and set fire to a match. Considerable exertion of the arm, however, is required to produce the latter result. This experiment affords an illustration of a general principle in nature, that all energy expended results either in a certain amount of work done or of heat produced. Accordingly, energy must be so directed as to produce the exact result desired. If we wish to produce heat, as in the case of the button, or in warming one's hands, the more energy that is applied to overcome the friction, the greater is the amount of heat produced. If sufficient energy be expended, the heat becomes so great that the rubbed bodies take fire. Savages, for example, light their fires by rubbing two sticks together; forests have been set on fire by the friction of two branches waving in the wind; and destructive fires have been occasioned by friction in a piece of machinery. More generally, however, energy is directed to the performance of work, and in this case all that goes to produce heat is lost. If, when a man is

sawing wood, the blade of the saw be held by the wood, the force required to overcome this friction, although it has the effect of heating the saw, is lost, because the object is not to heat the saw, but to cut the wood. To prevent this friction, the teeth of the saw are set outward to each side alternately, so as to make an opening wide enough to allow the blade to work freely; and sometimes a piece of wood is inserted in the cut, to keep the sides apart. When the friction cannot be altogether prevented, it is eased by rubbing the saw with grease. For the same reason, the axles of wheels of carts, railway carriages, and other machines, are kept carefully greased. (2) By Percussion.—On picking up a lead bullet, or rather the flattened fragment of one, just after it has struck a metal target, it is felt to be hot. The heat of the flattened ball is the exact equivalent of the force with which the bullet was moving when it struck the target, together with that communicated to the spot struck. Again, when a piece of cold iron is hammered, it becomes hot-that is, the energy expended in the blows is converted into heat in the iron, just as happens when a button is rubbed. (3) By Compression.-When the density of a body is increased by compression, heat is developed according as the volume of the body becomes diminished. When books are squeezed between the plates of a hydraulic press (see fig. 19, page 17), they are found to be heated; in other words, the force applied to the press has been converted into heat. Similarly, heat is evolved when a weight is laid on a metal pillar.

From a consideration of the foregoing and many similar facts, the conclusion has been arrived at, that heat is a form of motion. Thus, the heat produced by a bullet striking a target is simply the motion, which the bullet had before it struck the target, transferred to the atoms of the lead as well as to those of the metal struck; and the heat of the hammered iron is simply the motion of the hammer transferred to the atoms of the iron; and similarly in any case of friction or compression.

2. Change of Condition.-It must be distinctly understood that all bodies have a greater or less amount of heat. We are obliged to conceive of a point at which there is an entire absence of heat, but of that point we have no experience, and beyond it the heat of bodies differs merely in degree. The temperature of the human body is about 90°, and we are accustomed to speak of bodies with a lower temperature than this as cold, and of all having a higher temperature as warm or hot. Taking for granted, then, that heat is motion among the atoms of the body, let us consider how different bodies are affected by it. In all bodies, the atoms vibrate backward and forward, and these vibrations have greater or less velocity and extent, according to the amount of heat in the body. The result of these vibrations is that the atoms repel each other, so as to make the body composed of them, when heated to more than its ordinary temperature, occupy a larger space. Iron, for example, expands when heated, as was

« PredošláPokračovať »