250. I had two such plates mounted, one of copper, one of iron. The copper plate alone gave sixty vibrations, in the average of several experiments, before the arc of vibration was reduced from one constant mark to another.
On placing opposite magnetic poles near to, and on each side of, the same place, the vibrations were reduced to fifteen. On putting similar poles on each side of it, they rose to fifty; and on placing two pieces of wood of equal size with the poles equally near, they became fifty-two. So that, when similar poles were used, the magnetic effect was little or none, (the obstruction being due to the confinement of the air, rather,) whilst with opposite poles it was the greatest possible. When a pole was presented to the edge of the plate, no r.e.t.a.r.dation occurred.
251. The iron plate alone made thirty-two vibrations, whilst the arc of vibration diminished a certain quant.i.ty. On presenting a magnetic pole to the edge of the plate (247.), the vibrations were diminished to eleven; and when the pole was about half an inch from the edge, to five.
252. When the marked pole was put at the side of the iron plate at a certain distance, the number of vibrations was only five. When the marked pole of the second bar was put on the opposite side of the plate at the same distance (250.), the vibrations were reduced to two. But when the second pole was an unmarked one, yet occupying exactly the same position, the vibrations rose to twenty-two. By removing the stronger of these two opposite poles a little way from the plate, the vibrations increased to thirty-one, or nearly the original number. But on removing it _altogether_, they fell to between five and six.
253. Nothing can be more clear, therefore, than that with iron, and bodies admitting of ordinary magnetic induction, _opposite_ poles on opposite sides of the edge of the plate neutralize each other"s effect, whilst _similar_ poles exalt the action; a single pole end on is also sufficient.
But with copper, and substances not sensible to ordinary magnetic impressions, _similar_ poles on opposite sides of the plate neutralize each other; _opposite_ poles exalt the action; and a single pole at the edge or end on does nothing.
254. Nothing can more completely show the thorough independence of the effects obtained with the metals by Arago, and those due to ordinary magnetic forces; and henceforth, therefore, the application of two poles to various moving substances will, if they appear at all magnetically affected, afford a proof of the nature of that affection. If opposite poles produce a greater effect than one pole, the result will be due to electric currents. If similar poles produce more effect than one, then the power is _not_ electrical; it is not like that active in the metals and carbon when they are moving, and in most cases will probably be found to be not even magnetical, but the result of irregular causes not antic.i.p.ated and consequently not guarded against.
255. The result of these investigations tends to show that there are really but very few bodies that are magnetic in the manner of iron. I have often sought for indications of this power in the common metals and other substances; and once in ill.u.s.tration of Arago"s objection (82.), and in hopes of ascertaining the existence of currents in metals by the momentary approach of a magnet, suspended a disc of copper by a single fibre of silk in an excellent vacuum, and approximated powerful magnets on the outside of the jar, making them approach and recede in unison with a pendulum that vibrated as the disc would do: but no motion could be obtained; not merely, no indication of ordinary magnetic powers, but none or _any electric current_ occasioned in the metal by the approximation and recession of the magnet. I therefore venture to arrange substances in three cla.s.ses as regards their relation to magnets; first, those which are affected when at rest, like iron, nickel, &c., being such as possess ordinary magnetic properties; then, those which are affected when in motion, being conductors of electricity in which are produced electric currents by the inductive force of the magnet; and, lastly, those which are perfectly indifferent to the magnet, whether at rest or in motion.
256. Although it will require further research, and probably close investigation, both experimental and mathematical, before the exact mode of action between a magnet and metal moving relatively to each other is ascertained; yet many of the results appear sufficiently clear and simple to allow of expression in a somewhat general manner.--If a terminated wire move so as to cut a magnetic curve, a power is called into action which tends to urge an electric current through it; but this current cannot be brought into existence unless provision be made at the ends of the wire for its discharge and renewal.
257. If a second wire move in the same direction as the first, the same power is exerted upon it, and it is therefore unable to alter the condition of the first: for there appear to be no natural differences among substances when connected in a series, by which, when moving under the same circ.u.mstances relative to the magnet, one tends to produce a more powerful electric current in the whole circuit than another (201. 214.).
258. But if the second wire move with a different velocity, or in some other direction, then variations in the force exerted take place; and if connected at their extremities, an electric current pa.s.ses through them.
259. Taking, then, a ma.s.s of metal or an endless wire, and referring to the pole of the magnet as a centre of action, (which though perhaps not strictly correct may be allowed for facility of expression, at present,) if all parts move in the same direction, and with the same angular velocity, and through magnetic curves of constant intensity, then no electric currents are produced. This point is easily observed with ma.s.ses subject to the earth"s magnetism, and may be proved with regard to small magnets; by rotating them, and leaving the metallic arrangements stationary, no current is produced.
260. If one part of the wire or metal cut the magnetic curves, whilst the other is stationary, then currents are produced. All the results obtained with the galvanometer are more or less of this nature, the galvanometer extremity being the fixed part. Even those with the wire, galvanometer, and earth (170.), may be considered so without any error in the result.
261. If the motion of the metal be in the same direction, but the angular velocity of its parts relative to the pole of the magnet different, then currents are produced. This is the case in Arago"s experiment, and also in the wire subject to the earth"s induction (172.), when it was moved from west to east.
262. If the magnet moves not directly to or from the arrangement, but laterally, then the case is similar to the last.
263. If different parts move in opposite directions across the magnetic curves, then the effect is a maximum for equal velocities.
264. All these in fact are variations of one simple condition, namely, that all parts of the ma.s.s shall not move in the same direction across the curves, and with the same angular velocity. But they are forms of expression which, being retained in the mind, I have found useful when comparing the consistency of particular phenomena with general results.
_Royal Inst.i.tution, December 21, 1831._
THIRD SERIES.
-- 7. _Ident.i.ty of Electricities derived from different sources._ -- 8.
_Relation by measure of common and voltaic Electricity._
[Read January 10th and 17th, 1833.]
-- 7. _Ident.i.ty of Electricities derived from different sources._
265. The progress of the electrical researches which I have had the honour to present to the Royal Society, brought me to a point at which it was essential for the further prosecution of my inquiries that no doubt should remain of the ident.i.ty or distinction of electricities excited by different means. It is perfectly true that Cavendish[A], Wollaston[B], Colladon[C], and others, have in succession removed some of the greatest objections to the acknowledgement of the ident.i.ty of common, animal and voltaic electricity, and I believe that most philosophers consider these electricities as really the same. But on the other hand it is also true, that the accuracy of Wollaston"s experiments has been denied[D]; and also that one of them, which really is no proper proof of chemical decomposition by common electricity (309. 327.), has been that selected by several experimenters as the test of chemical action (336. 346.). It is a fact, too, that many philosophers are still drawing distinctions between the electricities from different sources; or at least doubting whether their ident.i.ty is proved. Sir Humphry Davy, for instance, in his paper on the Torpedo[E], thought it probable that animal electricity would be found of a peculiar kind; and referring to it, to common electricity, voltaic electricity and magnetism, has said, "Distinctions might be established in pursuing the various modifications or properties of electricity in those different forms, &c." Indeed I need only refer to the last volume of the Philosophical Transactions to show that the question is by no means considered as settled[F].
[A] Phil. Trans. 1779, p. 196.
[B] Ibid. 1801, p. 434.
[C] Annnles de Chimie, 1826, p. 62, &c.
[D] Phil. Trans. 1832, p. 282, note.
[E] Phil. Trans. 1892, p. 17.
"Common electricity is excited upon non-conductors, and is readily carried off by conductors and imperfect conductors. Voltaic electricity is excited upon combinations of perfect and imperfect conductors, and is only transmitted by perfect conductors or imperfect conductors of the best kind. Magnetism, if it be a form of electricity, belongs only to perfect conductors; and, in its modifications, to a peculiar cla.s.s of them[1]. Animal electricity resides only in the imperfect conductors forming the organs of living animals, &c."
[1] Dr. Ritchie has shown this is not the case. Phil. Trans. 1832, p.
294.
[F] Phil. Trans. 1832, p. 259. Dr. Davy, in making experiments on the torpedo, obtains effects the same as those produced by common and voltaic electricity, and says that in its magnetic and chemical power it does not seem to be essentially peculiar,--p. 274; but he then says, p. 275, there are other points of difference; and after referring to them, adds, "How are these differences to be explained?
Do they admit of explanation similar to that advanced by Mr. Cavendish in his theory of the torpedo; or may we suppose, according to the a.n.a.logy of the solar ray, that the electrical power, whether excited by the common machine, or by the voltaic battery, or by the torpedo, is not a simple power, but a combination of powers, which may occur variously a.s.sociated, and produce all the varieties of electricity with which we are acquainted?"
At p. 279 of the same volume of Transactions is Dr. Ritchie"s paper, from which the following are extracts: "Common electricity is diffused over the surface of the metal;--voltaic electricity exists within the metal. Free electricity is conducted over the surface of the thinnest gold leaf as effectually as over a ma.s.s of metal having the same surface;--voltaic electricity requires thickness of metal for its conduction," p. 280: and again, "The supposed a.n.a.logy between common and voltaic electricity, which was so eagerly traced after the invention of the pile, completely fails in this case, which was thought to afford the most striking resemblance." p. 291.
266. Notwithstanding, therefore, the general impression of the ident.i.ty of electricities, it is evident that the proofs have not been sufficiently clear and distinct to obtain the a.s.sent of all those who were competent to consider the subject; and the question seemed to me very much in the condition of that which Sir H. Davy solved so beautifully,--namely, whether voltaic electricity in all cases merely eliminated, or did not in some actually produce, the acid and alkali found after its action upon water.
The same necessity that urged him to decide the doubtful point, which interfered with the extension of his views, and destroyed the strictness of his reasoning, has obliged me to ascertain the ident.i.ty or difference of common and voltaic electricity. I have satisfied myself that they are identical, and I hope the experiments which I have to offer and the proofs flowing from them, will be found worthy the attention of the Royal Society.
267. The various phenomena exhibited by electricity may, for the purposes of comparison, be arranged under two heads; namely, those connected with electricity of tension, and those belonging to electricity in motion. This distinction is taken at present not as philosophical, but merely as convenient. The effect of electricity of tension, at rest, is either attraction or repulsion at sensible distances. The effects of electricity in motion or electrical currents may be considered as 1st, Evolution of heat; 2nd, Magnetism; 3rd, Chemical decomposition; 4th, Physiological phenomena; 5th, Spark. It will be my object to compare electricities from different sources, and especially common and voltaic electricities, by their power of producing these effects.
I. _Voltaic Electricity._
268. _Tension._--When a voltaic battery of 100 pairs of plates has its extremities examined by the ordinary electrometer, it is well known that they are found positive and negative, the gold leaves at the same extremity repelling each other, the gold leaves at different extremities attracting each other, even when half an inch or more of air intervenes.
269. That ordinary electricity is discharged by points with facility through air; that it is readily transmitted through highly rarefied air; and also through heated air, as for instance a flame; is due to its high tension. I sought, therefore, for similar effects in the discharge of voltaic electricity, using as a test of the pa.s.sage of the electricity either the galvanometer or chemical action produced by the arrangement hereafter to be described (312. 316.).
270. The voltaic battery I had at my disposal consisted of 140 pairs of plates four inches square, with double coppers. It was insulated throughout, and diverged a gold leaf electrometer about one third of an inch. On endeavouring to discharge this battery by delicate points very nicely arranged and approximated, either in the air or in an exhausted receiver, I could obtain no indications of a current, either by magnetic or chemical action. In this, however, was found no point of discordance between voltaic and common electricity; for when a Leyden battery (291.) was charged so as to deflect the gold leaf electrometer to the same degree, the points were found equally unable to discharge it with such effect as to produce either magnetic or chemical action. This was not because common electricity could not produce both these effects (307. 310.); but because when of such low intensity the quant.i.ty required to make the effects visible (being enormously great (371. 375.),) could not be transmitted in any reasonable time. In conjunction with the other proofs of ident.i.ty hereafter to be given, these effects of points also prove ident.i.ty instead of difference between voltaic and common electricity.
271. As heated air discharges common electricity with far greater facility than points, I hoped that voltaic electricity might in this way also be discharged. An apparatus was therefore constructed (Plate III. fig. 46.), in which AB is an insulated gla.s.s rod upon which two copper wires, C, D, are fixed firmly; to these wires are soldered two pieces of fine platina wire, the ends of which are brought very close to each other at _e_, but without touching; the copper wire C was connected with the positive pole of a voltaic battery, and the wire D with a decomposing apparatus (312. 316.), from which the communication was completed to the negative pole of the battery. In these experiments only two troughs, or twenty pairs of plates, were used.
272. Whilst in the state described, no decomposition took place at the point _a_, but when the side of a spirit-lamp flame was applied to the two platina extremities at _e_, so as to make them bright red-hot, decomposition occurred; iodine soon appeared at the point _a_, and the transference of electricity through the heated air was established. On raising the temperature of the points _e_ by a blowpipe, the discharge was rendered still more free, and decomposition took place instantly. On removing the source of heat, the current immediately ceased. On putting the ends of the wires very close by the side of and parallel to each other, but not touching, the effects were perhaps more readily obtained than before.
On using a larger voltaic battery (270.), they were also more freely obtained.
273. On removing the decomposing apparatus and interposing a galvanometer instead, heating the points _e_ as the needle would swing one way, and removing the heat during the time of its return (302.), feeble deflections were soon obtained: thus also proving the current through heated air; but the instrument used was not so sensible under the circ.u.mstances as chemical action.
274. These effects, not hitherto known or expected under this form, are only cases of the discharge which takes place through air between the charcoal terminations of the poles of a powerful battery, when they are gradually separated after contact. Then the pa.s.sage is through heated air exactly as with common electricity, and Sir H. Davy has recorded that with the original battery of the Royal Inst.i.tution this discharge pa.s.sed through a s.p.a.ce of at least four inches[A]. In the exhausted receiver the electricity would _strike_ through nearly half an inch of s.p.a.ce, and the combined effects of rarefaction and heat were such upon the inclosed air us to enable it to conduct the electricity through a s.p.a.ce of six or seven inches.
[A] Elements of Chemical Philosophy, p. 153
275. The instantaneous charge of a Leyden battery by the poles of a voltaic apparatus is another proof of the tension, and also the quant.i.ty, of electricity evolved by the latter. Sir H. Davy says[A], "When the two conductors from the ends of the combination were connected with a Leyden battery, one with the internal, the other with the external coating, the battery instantly became charged; and on removing the wires and making the proper connexions, either a shock or a _spark_ could be perceived: and the least possible time of contact was sufficient to renew the charge to its full intensity."
[A] Elements of Chemical Philosophy, p. 154.
276. _In motion:_ i. _Evolution of Heat._--The evolution of heat in wires and fluids by the voltaic current is matter of general notoriety.