Chap. X The Electrical Battery — And Experiments Performed With It
When a greater degree of electric force is required than a single jar is capable of giving, the electrical battery is made use of as part of the apparatus, which takes its name from the formidable effects it produces. This battery consists of a number of coated jars, placed in such a manner that they may all be charged at the same time, and discharged in an instant; so that the whole force of electricity accumulated in them, may at once be exerted on the substance exposed to the shock.
In discharging electrical jars, the electricity goes in the greatest quantity through the best conductors, and by the shortest passage. Thus if a chain and a wire be made to communicate at the same time with the outer coating of a jar, and be both presented to the knob of that jar, the greater part of the charge will pass by the wire, and very little by the chain, because the latter is a worse conductor than the former, on account of its discontinuation at every link. When the discharge is made by the chain only, sparks are seen at every link, which is a proof they are not in contact.
The force of an electric shock is not affected by the inflections of a conductor through which it passes, though it is sensibly weakened by its length. Hence, when the circuit or communication between the two sides of a Leyden phial is formed by one person applying his hands to the different sides, the shock is stronger than when it is formed by many persons joining hands. Yet a considerable shock was given by the Abbè Nollet, in the presence of the king of France, to one hundred and eighty men; who formed an electrical circuit.—They were all shocked in the same instant.
Doctor Watson and many other gentlemen of eminence in science, were at the pains of making experiments of the same kind. They found, by means of a wire insulated on baked wood, that the electric shock was transmitted instantaneously through the length of 12,276 feet.
Electricity transmitted in large quantities through living vegetables, destroys their vegetable life.
When transmitted, in the same form, through animals, it generally puts an end to animal life; though it is said that there are individuals who are not affected by it. Possibly the reason why some persons are not killed by very large electric shocks is, that their muscular system, or bodily organization, has something peculiar which protects them.
If an electrical circuit be made by means of imperfect conductors, as a slender piece of wood, a wet pack-thread, the discharge will be made silently.
If a small interruption of an electrical circuit be made in water, on making the discharge, a spark will be seen in the water, which never fails to agitate it and sometimes breaks the vessel in which it is contained.
A strong shock from a battery, sent through a slender piece of metal, instantly makes it red hot. Usually it is melted in whole or in part. If the fusion be perfect it is reduced into globules of different magnitudes. In this experiment it is a little remarkable that the parts of the metal at which the fluid enters and issues, are most likely to be melted.
If the metal be enclosed between pieces of glass, the shock will force the melted metal into the substance of the glass, so that it cannot afterwards be removed, without scraping off part of the glass with it. In this experiment the glasses which enclose metal are commonly broken to pieces.—It is seldom that they resist the force of a strong shock. If the glasses enclosing metal be pressed by a heavy weight, a small shock is often sufficient not only to raise the weight, but to break glasses of considerable thickness. When the pieces of glass are not broken, they are marked by the explosion with the most lively prismatic colours, which lie sometimes irregularly, and sometimes in their prismatic order.
Gun-powder may be fired by a charge from three square feet of coated glass. The powder is to be put into a quill, and then a wire is to be thrust into each end so as nearly to meet, and afterwards these wires are to be made a part of an electrical circuit.—A less charge of electricity will be sufficient if iron filings be mixed with the gun-powder.
When a shock somewhat less than is sufficient to melt a piece of metal is sent through a chain, a black dust, in the form of smoke, is seen to proceed from the chain. This dust is probably some of the metal itself, partly calcined, and by the violence of the explosion forced from it. If the chain be laid upon a piece of paper, glass, or other electric, this, after the explosion, will be found stained with some indelible marks, and often shew evident signs of having been burnt.
What is more remarkable in considering the effects of electricity on metals is, that it often, in a considerable degree, revivifies their calces or oxyds. In making experiments of this kind, the metallic calx or oxyd is to be made a part of an electrical circuit, through which a strong shock is to be sent: when the calx or oxyd will be found in a measure restored to its metallic state: the electric shock having, as it appears, taken away from the oxyd a portion of its oxygen.
The electric shock when passed through the magnetic needle, sometimes destroys its magnetic virtue, and sometimes reverses its poles. It is affirmed that two ships sailing together on the same voyage, were led, from the effect of lightning on their needles, to steer exactly opposite courses, after the storm in which they were exposed to the lightning had subsided. When the charge of ten, eight, or even a less number of square feet of coated glass, is sent through a sewing needle, it will often give it polarity, so that it will traverse when laid upon water. In this experiment it is remarkable that if the needle be lying east and west, that end of it which communicated with the positive coating will point towards the north; but if the needle be struck while lying north and south, that end of it which lay towards the north, will, in any case, point north; and the needle will acquire a stronger virtue in this than in the former case. But if the needle be placed perpendicular to the horizon, and the electric shock be given to either point of it, the lower extremity will afterwards point north.
The electric explosion taken upon the leaves of certain flowers changes their colour.
If the ball of a thermometer be placed in a strong current of electricity, the mercury or spirit will rise several degrees.
If a thin bottle be exhausted of air by means of an air pump, it will receive a considerable charge of electricity, by applying its bottom to an electrified prime conductor. In performing this experiment the bottle is to be held by the neck or near the mouth, and the electric matter will pass through the vacuum, and along the inner surface of the bottle, to the hand, from that end of it which is nearest to the prime conductor. The luminous appearance exhibited by this experiment is exceedingly beautiful in the dark, especially if the bottle be of any considerable length. It exactly resembles those lights which appear in the northern sky, and which are called streamers or the aurora borealis. If one hand be applied to the part of the bottle which was before presented to the prime conductor, while the other remains at the neck, a shock will be felt, at which instant the natural state of the inner surface is restored by a flash, which is seen pervading the vacuum between the two hands.—The principle on which this experiment depends will be explained hereafter.