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Instead of these two names, however, positive and negative are now more frequently used; and for convenience, the algebraic signs corresponding to these are employed, + and -. The sum of what has been said may now be stated thus: Bodies charged with either positive or negative electricity attract bodies charged with negative and positive electricity respectively, and attract bodies not electrified at all; but they repel all bodies charged with electricity of the same kind as their own; further, electricity can be communicated by contact from one body to another.

In the above experiment, a particular substance is rubbed with a particular kind of cloth, the glass with silk, and the sealing-wax with a woollen cloth. When electricity is produced in a substance by friction, electricity of the opposite kind is produced in the rubber or substance (a cloth generally) with which it is rubbed; and a body, which becomes charged with positive electricity when rubbed with one substance, may become charged with negative when rubbed with another. Thus, when glass is rubbed with silk, the electricity of the former is positive, and that of the latter, negative; but if the glass be rubbed with a cat's fur instead of silk, the electricity of the former is negative, and that of the latter, positive. In the following list, each body, when rubbed with the one before it in order, becomes charged with negative electricity; when with the one coming after it, with positive-cat's fur, glass, linen, feathers, wood, paper, silk, shell-lac, ground glass.

It must be remembered that all bodies do not become charged with electricity when rubbed; those that do are called electrics, and those that do not, non-electrics. There is another distinction to be noticed in bodies, and that is with regard to receiving the electricity which has been produced in an electric; those that do are called conductors, and those that do not, non-conductors. Most bodies are electrics, but it makes a material difference in this respect whether the body be a conductor or not; because, if the electric be also a conductor, the electricity produced is immediately carried off; while, if it be a non-conductor, the electricity remains on the surface, and is more apparent. Thus it happens that practically the most powerful electrics are non-conductors. The most important class of conductors are the metals. Water is also a good conductor. A few of the principal non-conductors, and thus, in a certain sense, the best electrics, are shell-lac, caoutchouc, amber, resin, sulphur, wax, glass, gems, silk, wool, hair, dry paper, leather, camphor, chalk, lime.

We can now explain why the pith-ball, made use of in the experiment, was suspended by a silk thread from a glass stand. Glass and silk are non-conductors, and so the electricity communicated to the pith-ball could not be conveyed away by these substances, as it would have been if it had been attached to a conductor; as, for instance, if it had been fastened by a

wire to an iron stand.

ground by being fixed insulated.

A conductor cut off from communication with the on non-conductors, in this way, is said to be

Let us now return to our first experiment. We saw by it that positive electricity attracted negative, and negative, positive; and also that both attracted bodies not electrified at all: it will now be shewn that the last part of this statement is the same as the first. If a brass cylinder be fixed on a glass stand, with a pair of pith-balls suspended by a cotton thread (not silk this time, because it is not desired to insulate them from the cylinder) at either end, and placed near another insulated conductor charged with positive electricity, this is what will happen-the balls will

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Fig. 33.

be repelled from each other, as in the figure, shewing that each pair has become charged with the same kind of electricity. But it is found that the electricity at the two ends is of different kinds, that in the end next the ball being negative, while that in the other end is positive. This is quite in accordance with the result of our first experiment, that positive electricity. attracts negative, and repels positive, and vice versa. From this it is manifest, that in all conductors there exist (without friction) the two kinds of electricity, and that these are acted upon by an electrified conductor exactly in the same way as if the electricity had been produced by friction in both. It will now be seen that, when the rubbed glass attracts the pith-ball the first time, the attraction is not different from the subsequent attractions; the positive electricity in the glass repels the positive in the ball to the opposite side, and attracts the negative. This attraction is great enough to overcome the slight weight of the ball; when contact takes place, the negative electricity in the side of the ball next the glass is replaced by positive electricity from the glass. The ball is then charged altogether with positive electricity, and of course is now repelled by the glass; but at the other side, the sealing-wax which is charged with negative electricity attracts it; and so on. The influence which an electrified conductor thus exercises on a non-electrified one is called Induction.

Lightning.

In the case of the pith-ball, when induction takes place, owing to its lightness and the manner in which it is suspended, it comes in contact with the glass, and electricity thus passes to it from the glass; but if, in fig. 33, there were a sufficient quantity of electricity in the ball, and the attraction were great enough, it would pass to the cylinder without contact; and there would be a slight flash of fire and a slight crack. Now, this flash

and this crack are lightning and thunder in miniature, for the black thunderclouds that we see in a thunder-storm are clouds charged with electricity. When two thunder-clouds approach each other, induction takes place, and owing to the immense quantity of electricity the two contain, when it passes from the one to the other, the flash is of exceeding brightness, and the report deafening. Sometimes the induction takes place between a cloud and a prominent object on the ground, such as a steeple, a tall chimney, a tree, or an animal. Anything will do, if it form a point near enough the cloud to cause the electricity to overcome the resistance of the intermediate space; and the consequences are generally fatal to the object in this position. But it is not necessarily so; the destructive effects of lightning, or of electricity rather, only take place when it meets with resistancethat is, when induction takes place between a thunder-cloud and some object which is a bad conductor of electricity. Hence the object of lightning-conductors. Metals being good conductors, if a rod of metal have its point extending beyond the top of any prominent object, and pass down into the ground, when a thunder-cloud happens to be near, and induction takes place, the electricity, which would otherwise have shivered the non-conducting steeple or chimney, passes harmlessly down the metal rod.

It may now be asked where this vast quantity of electricity in the thunder-cloud comes from. We saw that electricity is produced by friction; this, in the case of the thunder-cloud, is one source of it, for there are many different kinds of friction going on in nature. The friction produced by the wind in various ways is very great; thus electricity may be produced in a piece of glass by a blast of air from a bellows, instead of by rubbing. But, besides friction and all other forms of mechanical action, electricity is produced by all kinds of physical and chemical action; it is produced by the great system of evaporation which is continually going on from all bodies of water, and also from the processes of vegetation in which water is being continually separated and evaporated from plants, so that the vapours and gases that rise into the air are all more or less charged with electricity; and this electricity, which is generally diffused through the atmosphere, becomes occasionally concentrated in clouds, and is then liberated in the form of lightning.

The Electric Telegraph.

In the form of lightning, electricity is often very destructive, and is at all times very terrible; but the reason and skill of man have enabled him to turn it to account in a most useful manner. By means of the electric telegraph, a message can now be sent half round the world in a few hours.

We have said that one of the sources of electricity is chemical action;

electricity produced in this way is called Galvanism,1 or Voltaic2 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.

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Fig. 34.

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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. 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

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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.

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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.

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