Experimental Researches in Electricity Part 8

277. ii. _Magnetism._--No fact is better known to philosophers than the power of the voltaic current to deflect the magnetic needle, and to make magnets according to _certain laws_; and no effect can be more distinctive of an electrical current.

278. iii. _Chemical decomposition._--The chemical powers of the voltaic current, and their subjection to _certain laws_, are also perfectly well known.

279. iv. _Physiological effects._--The power of the voltaic current, when strong, to shock and convulse the whole animal system, and when weak to affect the tongue and the eyes, is very characteristic.

280. v. _Spark_.--The brilliant star of light produced by the discharge of a voltaic battery is known to all as the most beautiful light that man can produce by art.

281. That these effects may be almost infinitely varied, some being exalted whilst others are diminished, is universally acknowledged; and yet without any doubt of the ident.i.ty of character of the voltaic currents thus made to differ in their effect. The beautiful explication of these variations afforded by Cavendish's theory of quant.i.ty and intensity requires no support at present, as it is not supposed to be doubted.

282. In consequence of the comparisons that will hereafter arise between wires carrying voltaic and ordinary electricities, and also because of certain views of the condition of a wire or any other conducting substance connecting the poles of a voltaic apparatus, it will be necessary to give some definite expression of what is called the voltaic current, in contradistinction to any supposed peculiar state of arrangement, not progressive, which the wire or the electricity within it may be supposed to a.s.sume. If two voltaic troughs PN, P'N', fig. 42, be symmetrically arranged and insulated, and the ends NP' connected by a wire, over which a magnetic needle is suspended, the wire will exert no effect over the needle; but immediately that the ends PN' are connected by another wire, the needle will be deflected, and will remain so as long as the circuit is complete.

Now if the troughs merely act by causing a peculiar arrangement in the wire either of its particles or its electricity, that arrangement const.i.tuting its electrical and magnetic state, then the wire NP' should be in a similar state of arrangement _before_ P and N' were connected, to what it is afterwards, and should have deflected the needle, although less powerfully, perhaps to one half the extent which would result when the communication is complete throughout. But if the magnetic effects depend upon a current, then it is evident why they could not be produced in _any_ degree before the circuit was complete; because prior to that no current could exist.

283. By _current_, I mean anything progressive, whether it be a fluid of electricity, or two fluids moving in opposite directions, or merely vibrations, or, speaking still more generally, progressive forces. By _arrangement_, I understand a local adjustment of particles, or fluids, or forces, not progressive. Many other reasons might be urged in support of the view of a _current_ rather than an _arrangement_, but I am anxious to avoid stating unnecessarily what will occur to others at the moment.

II. _Ordinary Electricity._

284. By ordinary electricity I understand that which can be obtained from the common machine, or from the atmosphere, or by pressure, or cleavage of crystals, or by a mult.i.tude of other operations; its distinctive character being that of great intensity, and the exertion of attractive and repulsive powers, not merely at sensible but at considerable distances.

285. _Tension._ The attractions and repulsions at sensible distances, caused by ordinary electricity, are well known to be so powerful in certain cases, as to surpa.s.s, almost infinitely, the similar phenomena produced by electricity, otherwise excited. But still those attractions and repulsions are exactly of the same nature as those already referred to under the head _Tension, Voltaic electricity_ (268.); and the difference in degree between them is not greater than often occurs between cases of ordinary electricity only. I think it will be unnecessary to enter minutely into the proofs of the ident.i.ty of this character in the two instances. They are abundant; are generally admitted as good; and lie upon the surface of the subject: and whenever in other parts of the comparison I am about to draw, a similar case occurs, I shall content myself with a mere announcement of the similarity, enlarging only upon those parts where the great question of distinction or ident.i.ty still exists.

286. The discharge of common electricity through heated air is a well-known fact. The parallel case of voltaic electricity has already been described (272, &c.).

287. _In motion._ i. _Evolution of heat._--The heating power of common electricity, when pa.s.sed through wires or other substances, is perfectly well known. The accordance between it and voltaic electricity is in this respect complete. Mr. Harris has constructed and described[A] a very beautiful and sensible instrument on this principle, in which the heat produced in a wire by the discharge of a small portion of common electricity is readily shown, and to which I shall have occasion to refer for experimental proof in a future part of this paper (344.).

[A] Philosophical Transactions, 1827, p. 18. Edinburgh Transactions, 1831. Harris on a New Electrometer, &c. &c.

288. ii. _Magnetism._--Voltaic electricity has most extraordinary and exalted magnetic powers. If common electricity be identical with it, it ought to have the same powers. In rendering needles or bars magnetic, it is found to agree with voltaic electricity, and the _direction_ of the magnetism, in both cases, is the same; but in deflecting the magnetic needle, common electricity has been found deficient, so that sometimes its power has been denied altogether, and at other times distinctions have been hypothetically a.s.sumed for the purpose of avoiding the difficulty[A].

[A] Demonferrand's Manuel d'Electricite dynamique, p. 121.

289. M. Colladon, of Geneva, considered that the difference might be due to the use of insufficient quant.i.ties of common electricity in all the experiments before made on this head; and in a memoir read to the Academie des Sciences in 1826[A], describes experiments, in which, by the use of a battery, points, and a delicate galvanometer, he succeeded in obtaining deflections, and thus establis.h.i.+ng ident.i.ty in that respect. MM. Arago, Ampere, and Savary, are mentioned in the paper as having witnessed a successful repet.i.tion of the experiments. But as no other one has come forward in confirmation, MM. Arago, Ampere, and Savary, not having themselves published (that I am aware of) their admission of the results, and as some have not been able to obtain them, M. Colladon's conclusions have been occasionally doubted or denied; and an important point with me was to establish their accuracy, or remove them entirely from the body of received experimental research. I am happy to say that my results fully confirm those by M. Colladon, and I should have had no occasion to describe them, but that they are essential as proofs of the accuracy of the final and general conclusions I am enabled to draw respecting the magnetic and chemical action of electricity (360. 366. 367. 377. &c.).

[A] Annales de Chimie, x.x.xiii. p. 62.

290. The plate electrical machine I have used is fifty inches in diameter; it has two sets of rubbers; its prime conductor consists of two bra.s.s cylinders connected by a third, the whole length being twelve feet, and the surface in contact with air about 1422 square inches. When in good excitation, one revolution of the plate will give ten or twelve sparks from the conductors, each an inch in length. Sparks or flashes from ten to fourteen inches in length may easily be drawn from the conductors. Each turn of the machine, when worked moderately, occupies about 4/5ths of a second.

291. The electric battery consisted of fifteen equal jars. They are coated eight inches upwards from the bottom, and are twenty-three inches in circ.u.mference, so that each contains one hundred and eighty-four square inches of gla.s.s, coated on both sides; this is independent of the bottoms, which are of thicker gla.s.s, and contain each about fifty square inches.

292. A good _discharging train_ was arranged by connecting metallically a sufficiently thick wire with the metallic gas pipes of the house, with the metallic gas pipes belonging to the public gas works of London; and also with the metallic water pipes of London. It was so effectual in its office as to carry off instantaneously electricity of the feeblest tension, even that of a single voltaic trough, and was essential to many of the experiments.

293. The galvanometer was one or the other of those formerly described (87.

205.), but the gla.s.s jar covering it and supporting the needle was coated inside and outside with tinfoil, and the upper part (left uncoated, that the motions of the needle might be examined,) was covered with a frame of wire-work, having numerous sharp points projecting from it. When this frame and the two coatings were connected with the discharging train (292.), an insulated point or ball, connected with the machine when most active, might be brought within an inch of any part of the galvanometer, yet without affecting the needle within by ordinary electrical attraction or repulsion.

294. In connexion with these precautions, it may be necessary to state that the needle of the galvanometer is very liable to have its magnetic power deranged, diminished, or even inverted by the pa.s.sage of a shock through the instrument. If the needle be at all oblique, in the wrong direction, to the coils of the galvanometer when the shock pa.s.ses, effects of this kind are sure to happen.

295. It was to the r.e.t.a.r.ding power of bad conductors, with the intention of diminis.h.i.+ng its _intensity_ without altering its _quant.i.ty_, that I first looked with the hope of being able to make common electricity a.s.sume more of the characters and power of voltaic electricity, than it is usually supposed to have.

296, The coating and armour of the galvanometer were first connected with the discharging train (292.); the end B (87.) of the galvanometer wire was connected with the outside coating of the battery, and then both these with the discharging train; the end A of the galvanometer wire was connected with a discharging rod by a wet thread four feet long; and finally, when the battery (291.) had been positively charged by about forty turns of the machine, it was discharged by the rod and the thread through the galvanometer. The needle immediately moved.

297. During the time that the needle completed its vibration in the first direction and returned, the machine was worked, and the battery recharged; and when the needle in vibrating resumed its first direction, the discharge was again made through the galvanometer. By repeating this action a few times, the vibrations soon extended to above 40 on each side of the line of rest.

298. This effect could be obtained at pleasure. Nor was it varied, apparently, either in direction or degree, by using a short thick string, or even four short thick strings in place of the long fine thread. With a more delicate galvanometer, an excellent swing of the needle could be obtained by one discharge of the battery.

299. On reversing the galvanometer communications so as to pa.s.s the discharge through from B to A, the needle was equally well deflected, but in the opposite direction.

300. The deflections were in the same direction as if a voltaic current had been pa.s.sed through the galvanometer, i.e. the positively charged surface of the electric battery coincided with the positive end of the voltaic apparatus (268.) and the negative surface of the former with the negative end of the latter.

301. The battery was then thrown out of use, and the communications so arranged that the current could be pa.s.sed from the prime conductor, by the discharging rod held against it, through the wet string, through the galvanometer coil, and into the discharging train (292), by which it was finally dispersed. This current could be stopped at any moment, by removing the discharging rod, and either stopping the machine or connecting the prime conductor by another rod with the discharging train; and could be as instantly renewed. The needle was so adjusted, that whilst vibrating in moderate and small arcs, it required time equal to twenty-five beats of a watch to pa.s.s in one direction through the arc, and of course an equal time to pa.s.s in the other direction.

302. Thus arranged, and the needle being stationary, the current, direct from the machine, was sent through the galvanometer for twenty-five beats, then interrupted for other twenty-five beats, renewed for twenty-five beats more, again interrupted for an equal time, and so on continually. The needle soon began to vibrate visibly, and after several alternations of this kind, the vibration increased to 40 or more.

303. On changing the direction of the current through the galvanometer, the direction of the deflection of the needle was also changed. In all cases the motion of the needle was in direction the same as that caused either by the use of the electric battery or a voltaic trough (300).

304. I now rejected the wet string, and subst.i.tuted a copper wire, so that the electricity of the machine pa.s.sed at once into wires communicating directly with the discharging train, the galvanometer coil being one of the wires used for the discharge. The effects were exactly those obtained above (302).

305. Instead of pa.s.sing the electricity through the system, by bringing the discharging rod at the end of it into contact with the conductor, four points were fixed on to the rod; when the current was to pa.s.s, they were held about twelve inches from the conductor, and when it was not to pa.s.s, they were turned away. Then operating as before (302.), except with this variation, the needle was soon powerfully deflected, and in perfect consistency with the former results. Points afforded the means by which Colladon, in all cases, made his discharges.

306. Finally, I pa.s.sed the electricity first through an exhausted receiver, so as to make it there resemble the aurora borealis, and then through the galvanometer to the earth; and it was found still effective in deflecting the needle, and apparently with the same force as before.

307. From all these experiments, it appears that a current of common electricity, whether transmitted through water or metal, or rarefied air, or by means of points in common air, is still able to deflect the needle; the only requisite being, apparently, to allow time for its action: that it is, in fact, just as magnetic in every respect as a voltaic current, and that in this character therefore no distinction exists.

308. Imperfect conductors, as water, brine, acids, &c. &c. will be found far more convenient for exhibiting these effects than other modes of discharge, as by points or b.a.l.l.s; for the former convert at once the charge of a powerful battery into a feeble spark discharge, or rather continuous current, and involve little or no risk of deranging the magnetism of the needles (294.).

309. iii. _Chemical decomposition._--The chemical action of voltaic electricity is characteristic of that agent, but not more characteristic than are the _laws_ under which the bodies evolved by decomposition arrange themselves at the poles. Dr. Wollaston showed[A] that common electricity resembled it in these effects, and "that they are both essentially the same"; but he mingled with his proofs an experiment having a resemblance, and nothing more, to a case of voltaic decomposition, which however he himself partly distinguished; and this has been more frequently referred to by some, on the one hand, to prove the occurrence of electro-chemical decomposition, like that of the pile, and by others to throw doubt upon the whole paper, than the more numerous and decisive experiments which he has detailed.

[A] Philosophical Transactions, 1801, pp. 427, 434.

310. I take the liberty of describing briefly my results, and of thus adding my testimony to that of Dr. Wollaston on the ident.i.ty of voltaic and common electricity as to chemical action, not only that I may facilitate the repet.i.tion of the experiments, but also lead to some new consequences respecting electrochemical decomposition (376. 377.).

311. I first repeated Wollaston's fourth experiment[A], in which the ends of coated silver wires are immersed in a drop of sulphate of copper. By pa.s.sing the electricity of the machine through such an arrangement, that end in the drop which received the electricity became coated with metallic copper. One hundred turns of the machine produced an evident effect; two hundred turns a very sensible one. The decomposing action was however very feeble. Very little copper was precipitated, and no sensible trace of silver from the other pole appeared in the solution.

[A] Philosophical Transactions, 1801, p. 429.

312. A much more convenient and effectual arrangement for chemical decompositions by common electricity, is the following. Upon a gla.s.s plate, fig. 43, placed over, but raised above a piece of white paper, so that shadows may not interfere, put two pieces of tinfoil _a, b_; connect one of these by an insulated wire _c_, or wire and string (301.) with the machine, and the other _g_, with the discharging train (292.) or the negative conductor; provide two pieces of fine platina wire, bent as in fig. 44, so that the part _d, f_ shall be nearly upright, whilst the whole is resting on the three bearing points _p, e, f_ place these as in fig. 43; the points _p, n_ then become the decomposing poles. In this way surfaces of contact, as minute as possible, can be obtained at pleasure, and the connexion can be broken or renewed in a moment, and the substances acted upon examined with the utmost facility.

313. A coa.r.s.e line was made on the gla.s.s with solution of sulphate of copper, and the terminations _p_ and _n_ put into it; the foil _a_ was connected with the positive conductor of the machine by wire and wet string, so that no sparks pa.s.sed: twenty turns of the machine caused the precipitation of so much copper on the end _n_, that it looked like copper wire; no apparent change took place at _p_.

314. A mixture of equal parts of muriatic acid and water was rendered deep blue by sulphate of indigo, and a large drop put on the gla.s.s, fig. 43, so that _p_ and _n_ were immersed at opposite sides: a single turn of the machine showed bleaching effects round _p_, from evolved chlorine. After twenty revolutions no effect of the kind was visible at _n_, but so much chlorine had been set free at _p_, that when the drop was stirred the whole became colourless.

315. A drop of solution of iodide of pota.s.sium mingled with starch was put into the same position at _p_ and _n_; on turning the machine, iodine was evolved at _p_, but not at _n_.

316. A still further improvement in this form of apparatus consists in wetting a piece of filtering paper in the solution to be experimented on, and placing that under the points _p_ and _n_, on the gla.s.s: the paper retains the substance evolved at the point of evolution, by its whiteness renders any change of colour visible, and allows of the point of contact between it and the decomposing wires being contracted to the utmost degree.

A piece of paper moistened in the solution of iodide of pota.s.sium and starch, or of the iodide alone, with certain precautions (322.), is a most admirable test of electro-chemical action; and when thus placed and acted upon by the electric current, will show iodine evolved at _p_ by only half a turn of the machine. With these adjustments and the use of iodide of pota.s.sium on paper, chemical action is sometimes a more delicate test of electrical currents than the galvanometer (273.). Such cases occur when the bodies traversed by the current are bad conductors, or when the quant.i.ty of electricity evolved or transmitted in a given time is very small.

317. A piece of litmus paper moistened in solution of common salt or sulphate of soda, was quickly reddened at _p_. A similar piece moistened in muriatic acid was very soon bleached at _p_. No effects of a similar kind took place at _n_.