Heads of Lectures on a Course of Experimental Philosophy: Particularly Including Chemistry Part 15

It has not been found, that the electric shock takes up any sensible s.p.a.ce of time in being transmitted to the greatest distances.

The electric shock, as also the common electric spark, displaces the air through which it pa.s.ses; and if its pa.s.sage from conductor to conductor be interrupted by non-conductors of a moderate thickness, it will rend and tear them in its pa.s.sage, in such a manner as to exhibit the appearance of a sudden expansion of the air about the center of the shock.

If the electric circuit be interrupted, the electric matter, during the discharge, will pa.s.s to any other body that lies near its path, and instantly return. This may be called the _lateral explosion_. The effect of this lateral explosion through a bra.s.s chain, when the quant.i.ty of electricity is very great, will be the discolouring and partial burning of the paper on which it lies.

If a great quant.i.ty of electricity be acc.u.mulated, as in a _battery_, the explosion will pa.s.s over the surfaces of imperfect conductors without entering them, and the effect will be a strong _concussion_ of the substance. Also the electric matter thus acc.u.mulated and condensed will, by its repulsion, form _concentric circles_, which will appear by melting the surface of a flat piece of metal on which the explosion is received.

If an electric shock, or strong spark, be made to pa.s.s through, or over, the belly of a muscle, it forces it to contract, as in a convulsion.

If a strong shock be sent through a small animal body, it will often deprive it instantly of life.

When the electric shock is very strong, it will give polarity to magnetic needles, and sometimes it reverses their poles.

Great shocks, by which animals are killed, are said to hasten putrefaction.

Electricity and lightning are in all respects the same thing; since every effect of lightning may be imitated by electricity, and every experiment in electricity may be made with lightning, brought down from the clouds by means of insulated pointed rods of metal.

LECTURE x.x.xVI.

_The same Subject continued._

Three curious and important instruments, which are among the latest improvements in electricity, deserve a particular explanation, and in all of them the effect depends upon the general principles mentioned above, viz. that bodies placed within the influence, or, as it is usually termed, within the atmosphere, of an electrified body, are affected by a contrary electricity, and that these two electricities mutually attract each other. These instruments are the _electrophorus_, the _condenser_ of electricity, and the _doubler_ of it.

The electrophorus consists of an insulated conducting plate applied to an insulated electric. If the latter have any electricity communicated to it, for example the negative, the positive electricity of the former will be attracted by it, and consequently the plate will be capable of receiving electricity from any body communicating with the earth; being, in this situation, capable of containing more electricity than its natural quant.i.ty. Consequently, when it is removed from the lower plate, and the whole of its electricity equally diffused through it, it will appear to have a redundance, and therefore will give a spark to any body communicating with the earth. Being then replaced upon the electric, and touched by any body communicating with the earth, it will be again affected as before, and give a spark on being raised; and this process may be continued at pleasure, the electrophorus supplying the place of any other electrical machine.

If the conducting plate of the electrophorus be applied to a piece of dry wood, marble, or any other substance through which electricity can pa.s.s but very slowly, or if the insulated conducting plate be covered with a piece of thin silk, which will make some resistance to the pa.s.sage of electricity, and it be then applied to another plate communicating with the earth; and if, in either of these cases, a body with a large surface possessed of a weak electricity be applied to the conducting plate, the weak electricity not being able to overcome the obstruction presented to it, so as to be communicated to the other plate, will affect it with the contrary electricity, and this reacting on the first plate, will condense its electricity on that part of the plate to which it is contiguous; in consequence of which its capacity of receiving electricity will be increased; so that on the separation of the two plates, that electricity which was before condensed, being equally diffused through the whole plate, will have a greater intensity than it had before, attracting light bodies, or even giving a spark, when the body from which it received its electricity was incapable of it. For though it contained a great quant.i.ty of electricity, it was diffused through so large a s.p.a.ce that its intensity was very small.

This instrument is therefore called a _condenser of electricity_.

If an insulated plate of metal possessing the smallest degree of electricity be presented very near to another plate communicating with the earth, it will affect this plate with the opposite electricity; and this being, in the same manner, applied to a third plate, will put it into the same state with the first. If then these two plates be joined, and the first plate be presented to either of them, its own electricity being attracted by that of the plate presented, that of the other will be drawn into it, so that its quant.i.ty will be doubled. The same process being repeated, will again double the electricity of this plate, till, from being quite insensible to the most exquisite electrometer, it will become very conspicuous, or even give sparks. This instrument is therefore called a _doubler of electricity_, of excellent use in ascertaining the quality of atmospherical electricity when ever so small. If this instrument be so constructed that these three plates can be successively presented to one another by the revolution of one of them on an axis, it is called the _revolving doubler_; and in this form it is most convenient for use.

THE END.