Conversations on Chemistry Part 26

Certainly not, until the two are fully charged; for the two conductors will receive equal quant.i.ties of electricity.

CAROLINE.

I thought the use of the chain had been to convey the electricity _from_ the ground into the machine?

MRS. B.

That was the idea of Dr. Franklin, who supposed that there was but one kind of electricity, and who, by the terms positive and negative (which he first introduced), meant only different quant.i.ties of the same kind of electricity. The chain was in that case supposed to convey electricity _from_ the ground through the rubber into the conductor. But as we have adopted the hypothesis of two electricities, we must consider the chain as a vehicle to conduct the negative electricity into the earth.

EMILY.

And are both kinds of electricity produced whenever electricity is excited?

MRS. B.

Yes, invariably. If you rub a tube of gla.s.s with a woollen cloth, the gla.s.s becomes positive, and the cloth negative. If, on the contrary, you excite a stick of sealing-wax by the same means, it is the rubber which becomes positive, and the wax negative.

But with regard to the Voltaic battery, in order that the acid may act freely on the zinc, and the two electricities be given out without interruption, some method must be devised, by which the plates may part with their electricities as fast as they receive them. --Can you think of any means by which this might be effected?

EMILY.

Would not two chains or wires, suspended from either plate to the ground, conduct the electricities into the earth, and thus answer the purpose?

MRS. B.

It would answer the purpose of carrying off the electricity, I admit; but recollect, that though it is necessary to find a vent for the electricity, yet we must not lose it, since it is the power which we are endeavouring to obtain. Instead, therefore, of conducting it into the ground, let us make the wires, from either plate, meet: the two electricities will thus be brought together, and will combine and neutralize each other; and as long as this communication continues, the two plates having a vent for their respective electricities, the action of the acid will go on freely and uninterruptedly.

EMILY.

That is very clear, so far as two plates only are concerned; but I cannot say I understand how the energy of the succession of plates, or rather pairs of plates, of which the Galvanic trough is composed, is propagated and acc.u.mulated throughout a battery?

MRS. B.

In order to shew you how the intensity of the electricity is increased by increasing the number of plates, we will examine the action of four plates; if you understand these, you will readily comprehend that of any number whatever. In this figure (PLATE VI. Fig. 4.), you will observe that the two central plates are united; they are soldered together, (as we observed in describing the Voltaic trough,) so as to form but one plate which offers two different surfaces, the one of copper, the other of zinc.

Now you recollect that, in explaining the action of two plates, we supposed that a quant.i.ty of electricity was evolved from the surface of the first zinc plate, in consequence of the action of the acid, and was conveyed by the interposed fluid to the copper plate, No. 2, which thus became positive. This copper plate communicates its electricity to the contiguous zinc plate, No. 3, in which, consequently, some acc.u.mulation of electricity takes place. When, therefore, the fluid in the next cell acts upon the zinc plate, electricity is extricated from it in larger quant.i.ty, and in a more concentrated form, than before. This concentrated electricity is again conveyed by the fluid to the next pair of plates, No. 4 and 5, when it is farther increased by the action of the fluid in the third cell, and so on, to any number of plates of which the battery may consist; so that the electrical energy will continue to acc.u.mulate in proportion to the number of double plates, the first zinc plate of the series being the most negative, and the last copper plate the most positive.

CAROLINE.

But does the battery become more and more strongly charged, merely by being allowed to stand undisturbed?

MRS. B.

No, for the action will soon stop, as was explained before, unless a vent be given to the acc.u.mulated electricities. This is easily done, however, by establishing a communication by means of the wires (Fig. 1.), between the two ends of the battery: these being brought into contact, the two electricities meet and neutralize each other, producing the shock and other effects of electricity; and the action goes on with renewed energy, being no longer obstructed by the acc.u.mulation of the two electricities which impeded its progress.

EMILY.

Is it the union of the two electricities which produces the electric spark?

MRS. B.

Yes; and it is, I believe, this circ.u.mstance which gave rise to Sir H.

Davy's opinion that caloric may be a compound of the two electricities.

CAROLINE.

Yet surely caloric is very different from the electrical spark?

MRS. B.

The difference may consist probably only in intensity: for the heat of the electric spark is considerably more intense, though confined to a very minute spot, than any heat we can produce by other means.

EMILY.

Is it quite certain that the electricity of the Voltaic battery is precisely of the same nature as that of the common electrical machine?

MRS. B.

Undoubtedly; the shock given to the human body, the spark, the circ.u.mstance of the same substances which are conductors of the one being also conductors of the other, and of those bodies, such as gla.s.s and sealing-wax, which are non-conductors of the one, being also non-conductors of the other, are striking proofs of it. Besides, Sir H.

Davy has shewn in his Lectures, that a Leyden jar, and a common electric battery, can be charged with electricity obtained from a Voltaic battery, the effect produced being perfectly similar to that obtained by a common machine.

Dr. Wollaston has likewise proved that similar chemical decompositions are effected by the electric machine and by the Voltaic battery; and has made other experiments which render it highly probable, that the origin of both electricities is essentially the same, as they show that the rubber of the common electrical machine, like the zinc in the Voltaic battery, produces the two electricities by combining with oxygen.

CAROLINE.

But I do not see whence the rubber obtains oxygen, for there is neither acid nor water used in the common machine, and I always understood that the electricity was excited by the friction.

MRS. B.

It appears that by friction the rubber obtains oxygen from the atmosphere, which is partly composed of that element. The oxygen combines with the amalgam of the rubber, which is of a metallic nature, much in the same way as the oxygen of the acid combines with the zinc in the Voltaic battery, and it is thus that the two electricities are disengaged.

CAROLINE.

But, if the electricities of both machines are similar, why not use the common machine for chemical decompositions?

MRS. B.

Though its effects are similar to those of the Voltaic battery, they are incomparably weaker. Indeed Dr. Wollaston, in using it for chemical decompositions, was obliged to act upon the most minute quant.i.ties of matter, and though the result was satisfactory in proving the similarity of its effects to those of the Voltaic battery, these effects were too small in extent to be in any considerable degree applicable to chemical decomposition.

CAROLINE.

How terrible, then, the shock must be from a Voltaic battery, since it is so much more powerful than an electrical machine!

MRS. B.

It is not nearly so formidable as you think; at least it is by no means proportional to the chemical effect. The great superiority of the Voltaic battery consists in the large _quant.i.ty_ of electricity that pa.s.ses; but in regard to the _rapidity_ or _intensity_ of the charge, it is greatly surpa.s.sed by the common electrical machine. It would seem that the shock or sensation depends chiefly upon the intensity; whilst, on the contrary, for chemical purposes, it is quant.i.ty which is required. In the Voltaic battery, the electricity, though copious, is so weak as not to be able to force its way through the fluid which separates the plates, whilst that of a common machine will pa.s.s through any s.p.a.ce of water.

CAROLINE.