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 quantity. 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 pass but very slowly, or if the insulated conducting plate be covered with a piece of thin silk, which will make some resistance to the passage 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 quantity of electricity, it was diffused through so large a space 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 quantity 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.
[1] One gentleman of the Roman Catholic persuasion, and several of the Church of England, are now in the College.