Page images
[ocr errors]

quite independent of the extent of the edge, the area being the same; for Sir W. S. Harris found that the electrical intensity of a charged sphere is the same with that of a plane circular area of the same superficial extent, and that of a charged cylinder the same as if it were cut open and expanded into a plane surface.

The same able electrician has shown that the attractive force between an electrified and a neutral uninsulated body is the same whatever be the forms of their unopposed parts. Thus two hemispheres attract each other with precisely the same force as if they were spheres; and as the force is as the number of attracting points in operation directly, and as the squares of the respective distances inversely, it follows that the attraction between a mere ring and a circular area is no greater than that between two similar rings, and the force between a sphere and an opposed spherical segment of the same curvature is no greater than that of two similar segments, each equal to the given segment.

Electricity may be accumulated to a great extent in insulated bodies, and so long as it is quiescent it occasions no sensible change in their properties. When restrained by the non-conducting power of the atmosphere, its tension or the pressure it exerts is proportional to the coercive force of the air. If the pressure be less than the coercive force, the electricity is retained; but the instant it exceeds that force in any one point it escapes, and that more readily when the air is attenuated or saturated with moisture, for the resistance of the air is proportional to the square of its density, but the inductive action of electricity on distant bodies is independent of atmospheric pressure. The power of retaining electricity depends also on the shape of the charged body. It is most easily retained by a sphere, next to that by a spheroid, but it readily escapes from a point, and a pointed object receives it with most facility.

The heat produced by the electric shock is proportional to the square of the quantity of electricity discharged, and is so intense that it fuses metals and volatilizes substances, but its intensity is not felt to its full extent on account of the shortness of its duration. It is only accompanied by light when the electricity is obstructed in its passage through substance.

Electrical light when analysed by a prism differs very much

from solar light. Fraunhofer found that, instead of the fixed dark lines, the spectrum of an electric spark is crossed by numerous bright lines; and Professor Wheatstone has observed that the number and position of the lines differ with the metal from which the spark is taken, and believes the spark itself results from the ignition and volatilization of the matter of the conductor.

According to the experiments of Sir Humphry Davy, the density of the air has an influence on the colour. He passed the electric spark through a vacuum over mercury, which from green became successively sea-green, blue, and purple, on admitting different quantities of air. When the vacuum was made over a fusible alloy of tin and bismuth; the spark was yellowish and extremely pale. Sir Humphry thence concluded that electrical light principally depends upon some properties belonging to the ponderable matter through which it passes, and that space is capable of exhibiting luminous appearances, though it does not contain an appreciable quantity of matter. He thought that the superficial particles of bodies which form vapour, when detached by the repulsive power of heat, might be equally separated by the electric forces, and produce luminous appearances in vacuo by the destruction of their opposite electric states.

The velocity of electricity is so great that the most rapid motion which can be produced by art appears to be actual rest when compared with it. A wheel revolving with celerity sufficient to render its spokes invisible, when illuminated by a flash of lightning, is seen for an instant with all its spokes distinct, as if it were in a state of absolute repose; because, however rapid the rotation may be, the light has come and already ceased before the wheel has had time to turn through a sensible space. This beautiful experiment is due to Professor Wheatstone, as well as the following variation of it, which is not less striking: If a circular piece of pasteboard be divided into three sectors, one of which is painted blue, another yellow, and a third red, it will appear to be white when revolving quickly, because of the rapidity with which the impressions of the colours succeed each other on the retina. But, the instant it is illuminated by an electric spark, it seems to stand still, and each colour is as distinct as if it were at rest. This transcendent speed of electricity has

been ingeniously measured, as follows, by Professor Wheatstone, who has ascertained that it much surpasses the velocity of light.

In the horizontal diameter of a small disc, fixed on the wall of a darkened room, are disposed six small brass balls, well insulated from each other. An insulated copper wire, half a mile long, is disjointed in its middle, and also near its two extremities; the six ends thus obtained are connected with the six balls on the disc. When an electric discharge is sent through the wire by connecting its two extremities, one with the positive, and the other with the negative coating of a Leyden jar, three sparks are seen on the disc, apparently at the same instant. At the distance of about ten feet a small revolving mirror is placed so as to reflect these three sparks during its revolution. From the extreme velocity of the electricity, it is clear that, if the three sparks be simultaneous, they will be reflected, and will vanish before the mirror has sensibly changed its position, however rapid its rotation may be, and they will be seen in a straight line. But if the three sparks be not simultaneously transmitted to the disc—if one, for example, be later than the other two-the mirror will have time to revolve through an indefinitely small arc in the interval between the reflection of the two sparks and that of the single one. However, the only indication of this small motion of the mirror will be, that the single spark will not be reflected in the same straight line with the other two, but a little above or below it, for the reflection of all three will still be apparently simultaneous, the time intervening being much too short to be appreciated.

Since the number of revolutions which the revolving mirror makes in a second is known, and the angular deviation of the reflection of the single spark from the reflection of the other two can be measured, the time elapsed between their consecutive reflections can be ascertained. And, as the length of that part of the wire through which the electricity has passed is given, its velocity may be found.

The number of pulses in a second, requisite to produce a musical note of any pitch, are known; hence the number of revolutions accomplished by the mirror in a given time may be determined from the musical note produced by a tooth or peg, in its axis of

rotation, striking against a card, or from the notes of a siren attached to the axis. It was thus that Professor Wheatstone found the mirror which he employed in his experiments made 800 revolutions in a second; and, as the angular velocity of the reflected image in a revolving mirror is double that of the mirror itself, an angular deviation of one degree in the appearance of the two sparks would indicate an interval of the 576,000th of a second; the deviation of half a degree would, therefore, indicate more than the millionth of a second. The use of sound as a measure of velocity is a happy illustration of the connexion of the physical sciences.

The earth possesses a powerful electrical tension, and the atmosphere when clear is almost always positively electric. Its electricity is stronger in winter than in summer, during the day than in the night. The intensity increases for two or three hours from the time of sunrise, comes to a maximum between seven and eight, then decreases towards the middle of the day, arrives at its minimum between one and two, and again augments as the sun declines till about the time of sunset, after which it diminishes and continues, feeble during the night. The mere condensation of vapour is a source of atmospheric electricity ; but although it is also produced by the vapour that rises from the surface of the earth, it is not under all circumstances. M. Pouillet found that electricity is only developed when accompanied by chemical action: for example, when the water whence the vapour proceeds contains lime, chalk, or any solid alkali, negative electricity is produced; and when it holds in solution either gas, acid, or some of the salts, the vapour is positively electric. Besides, the contact of earth with salt and fresh water generates positive electricity, and the contact of fresh and salt currents of water negative, so that the ocean must afford a great supply to the atmosphere; hence thunderstorms are most frequent near the coasts: but as electricity of one kind or another is developed whenever the molecules of matter are deranged from their natural state of equilibrium, there must be many partial variations in the electric state of the air. When the invisible vapour rises charged with electricity into the cold regions of the atmosphere, it is condensed into cloud, in which the tension is increased because the electricity is confined to a smaller space; and if the conden

sation be sufficient to produce drops of rain, they carry the electricity to the ground, so that in general a shower is a conductor between the clouds and the earth. When two clouds charged with opposite kinds, but of equal tension, approach within a certain distance, the intensity increases on the sides of the clouds that are nearest to one another; and when the tension is great enough to overcome the coercive pressure of the atmosphere, a discharge takes place which causes a flash of lightning, the stroke being given either by the cloud or the rain. The actual quantity of electricity in any part of a cloud is extremely small. The intensity of the flash arises from the great extent of surface over which it is spread, so that clouds may be compared to enormous Leyden jars thinly coated with electricity, which only acquires its intensity by its instantaneous condensation. The rapid and irregular motions of thunder clouds are probably more owing to strong electrical attractions and repulsions among themselves than to currents of air, though both are no doubt concerned in these hostile movements. The atmosphere becomes intensely electric on the approach of rain, hail, snow, sleet, and wind; but it varies afterwards, and the transitions are very rapid on the approach of a thunderstorm.

In a

Since air is a non-conductor, it does not convey the electricity from the clouds to the earth, but it acquires from them an opposite kind, and when the tension is very great the force of the electricity becomes irresistible, and an interchange takes place between the clouds and the earth; but so rapid is the motion of lightning, that it is difficult to ascertain whether it goes from the clouds to the earth or shoots upwards from the earth to the clouds, though there can be no doubt that it does both. storm that occurred at Manchester in June 1835, the lightning was observed to issue from various points of a road, attended by explosions as if pistols had been fired out of the ground, and a man seems to have been killed by one of these explosions taking place under his foot. M. Gay Lussac ascertained that a flash of lightning sometimes darts more than three miles in a straight line. A person may be killed by lightning, although the explosion takes place at a distance of twenty miles, by what is called the back stroke. Suppose that the two extremities of a highly charged cloud hang down towards the earth, they will repel the

« PreviousContinue »