Abbildungen der Seite
PDF
EPUB

valuable for accurate measurements, when the wire cylinders are so contrived that they for a counterpoise, are enabled to make the necessary conditions of equilibrium, which is attained when the wire cylinder can so act as to repel the magnetic bar. For this purpose, one of the cylinders must be fastened below that bar, and the other over it. Through the latter hollow cylinder passes a pin or wire, by means of which the magnetic bar is suspended from the arm of the balance. It is necessary, however, to apply a correction to the indications afforded by this instrumental measurement of the powers of the currents. This correction, the necessity of which has been shewn by other contemporaneous labours, is in the proportion of the squares of the powers of the currents. If, for instance, the absolute power of the current be represented by k’, and the power indicated by the measuring instrument by k, we shall have this equation:

1 k=k-y k”, from which we obtain k'= (1--1 1-4 k y) From a series of observations, I found for my equipoise

y =

= 0.00004228.* The following table contains the experiments made with the before-mentioned voltaic series. The first column shews the resistance L, offered by the connecting wire which served as the spiral, a resistance which was ascertained by other experiments.

The other two columns contain the true powers of the currents ; that is, the measured powers, corrected by the above formula :

2 y

[blocks in formation]

If A and A' represent the electromotive powers, and 1 and X' the resistances presented by the series themselves, we shall have, according to the formula of Ohm, these four equationsA

A' = 380

= 395 at 23:1

x + 23:1

= 97

A

A'

= 135
+ 135:3

' + 135:3
Therefore
SA = 14610

= 23,000
= 15.35

35 * Poggendorff's Annalen, Bd. xlvii., s. 225. + We propose to give Professor Ohm's Theory of Electric Currents in an early number,--Edit.

B2

X =

[ocr errors]

Or, if we take 1, which, in this case, is 36 square inches, for the unit of surface,

35 X 2.5

= 2:4

36 If s represent the whole surface of the pile, z the number of pairs, c the strength of the current, L any

resistance,

z As Then, C=

z + Ls From this equation, we learn that the maximum of power of the current is obtained, when the pile is so managed, that = L; that is, when the whole resistance of the pile is equal to that introduced into the pile itself, in addition to the foreign resistance of the conductors. Hence, for any other arrangement than that which answers to a maximum of action, no constant relation between the different voltaic combinations are extant; nor can we, under any other circumstances, form a correct judgment of their relative superiority. By the elimination of z we have—

[ocr errors]

S

[ocr errors]

AV's

:

C (max.) =
2 XL

2 VAL from which, after substitution of the former for A, A', 1, X', their values are found to be

For the whole surface s'=s 0:06,

And in reference to the number of pairs z' = z 0:6; that is, we need only a pile of six square feet of platinum surface, in order to supply the place of a pile of a hundred square feet of copper surface; or, in reference to the number of pairs :—Six pairs of platinum, of a square foot of surface each, give the same action as ten pairs of copper of ten square feet of surface each. This extraordinary superiority of platinum has been well confirmed by other experiments.*

* At the time I received the above pote, its author sent me a leaf of the Journal de St. Petersburg, in which he states that the electro-magnetic powers of a Grove's battery would become applicable to navigation. The battery of which he speaks, consists of 64 pairs of platinum plates, each plate 36 square inches of surface; so that the whole platinum surface is 16 square feet. This is probably the mightiest battery that has hitherto been constructed, the great battery of the London Institution, of 2000 pairs of plates, and 1000 square feet of surface, not excepted. Notwithstanding the extraordinary powers of its thermo-chemical and dynamic action, the physiological effects of this great battery, as shown by Schönbein's small one of six pairs of plates, are only feeble. The action on the human body is discernible, but is easy to bear. We must remark, in this place, that the author does not look upon this phenomenon as a specific quality of the platinum battery, nor as an anomaly; since it finds an explauation in Prof. Ohm's mathematical laws of the galvanic battery.- Poggendurff.

[ocr errors]

On the Observations of M. BecQUEREL, relative to the Comparative

Measures of Action of two Voltaic Series; the one Copper-zinc, the other Platinum-zinc. By Dr. M. H. Jacobi, Mem. Imp. Academy St. Petersburg, &c. &c. At the sitting of January 13th, of last year, I had the honour of presenting a note to the Academy, on the measurement of the comparative action of two distinct voltaic series, the copper-zinc, and the platinum-zinc. This note has given rise to some observations by M. Bequerel, at the Academy of Sciences, Paris, on the 4th January last, in which he thus expresses himself :

" It seems from this conclusion, that in piles in general, platinum is far superior to copper for transmitting large quantities of electricity. But this superiority, in the present case, due to a condition which M. Jacobi has not noticed, although it exercises a considerable influence on the action of the pile, which is sometimes neglected by experimentalists."

M. Becquerel attributes the superiority of the platinum series to the reaction of the concentrated nitric acid upon the acidulated water, which gives rise to an electric current, of much more considerable energy than that resulting from the solution of sulphate of of copper on the similarly acidulated water in the copper series.

M. Becquerel then adds, “I thought it my duty to make these observations, which in other respects are not new, in order that experimenters may not be led into error respecting the causes of the effects obtained by M. Jacobi.”

From this observation, one might suppose that, in my note, there was a very serious omission, since M. Becquerel thought it his duty to fill it up

The calculations from my experimental data were made accordingly with the formula of Ohm; and the result shows, clearly enough, the cause of the effects : but to this formula I have thought proper to introduce the expression of the maximum of effect, because it is this expression (till then not noticed) that places us in circumstances which enable us to make accurate comparisons on the action of different voltaic combinations. It is with pleasure that I embrace this opportunity of declaring myself in favour of the theory of M. Ohm, which, in my opinion, is one of the most extraordinary advances in this department of natural philosophy. It collects into one view an immense assemblage of facts; and explains distinctly the whole of the phenomena which relate to the force of voltaic currents. Indeed, this theory has been confirmed and extended by some of the most distinguished philosophers. The law of Ohm, known in Germany for more than thirteen years, is now beginning to spread itself over England, and will give, I hope, a new direction to the zealous experimentalists of that country. In France, the same law, ten years after its publication, was announced as a new discovery, and has been received as such.

Respecting the point at issue, scarcely knowing in what manner to submit to calculation the chemical reaction of which M. Becquerel speaks, I have contented myself, according to observations, to account for the superiority of Mr. Grove's battery.

1. Because, in the same transverse section, the resistance in this pile is to that of the copper-zinc as 2.4 15.35; and

2. Because, under the conditions in question, the electro-motive force of platinum-zinc is to that of copper-zinc as 23000 to 14610.

These numbers, which result from the relation of resistances, do not present any thing remarkable, as concentrated nitric acid has long been known as one of the best conductors, and the resistance to transmission is lessened when the metals are immersed in strong acids.

As to the electromotive force, excited principally by the contact of two heterogeneous metals, there enters, without doubt, in part, the force resulting from the contact of the two heterogeneous liquids, or, if you please, from their mutual chemical reactions. The existence of such an action has, for some time, been questioned, and it is some recent experiments only that has placed it beyond doubt. But this is not what M. Becquerel seems to allude to, for it may be shewn that this latter effect is so inconsiderable, that the superiority of the platinum-zinc might rather be attributed to any other cause. The very opposite opinion to that entertained by M. Becquerel, appears to be favoured by the pile that he himself has discovered, and which now bears his name. This pile is well known to be composed of two platinum plates, one immersed in concentrated nitric acid, and the other in a solution of caustic potash.* Here are two metals, in appearance the same, and the circumstances are the most favourable for chemical reaction between the nitre acid and the alkali, which, without doubt, is far more powerful than that between nitric acid and diluted sulphuric acid. But M. Fechner has proved, by the most careful experiments, instituted without prejudice, and with the skill and accuracy which characterise all his operations, that if we represent the total force of a pile of this construction, by the number 8:644, the share of this force, due to reaction between the acid and alkali, will only be 0.140, or about a sixtieth part of the whole. This numerical result appears rather unfavourable to M. Becquerel's explanation, and will be still more so if we compare the action where the platinum is placed in the alkaline solution, with that which is brought into play where the zinc is placed in the acidulated water. It is to be hoped that M. Fechner, who is dexterous in the manipulation of these experiments, and who is possessed of the necessary apparatus, will examine this business still further. It appears to me, from the present state of our knowledge, that

We belive that Sir H. Davy was the first philosopher who employed piles or batteries of this kind.--Epit.

[ocr errors][ocr errors]

chemical action can scarcely be admitted as the only source of voltaic phenomena ; nevertheless, it is one of the chief agents, * and I am not afraid to state, briefly, some well established facts, which, to me appear to explain in what that agency consists :

1. A voltaic current can only exist by the contact of heterogeneous metals, or, in general, by the contact of dissimilar bodies.

2. Limiting our reasoning to metals—a class of bodies from which we derive the most unequivocal results—there can be no pair of them, unless perfectly homogeneous, that will not produce an electric current when immersed in one and the same liquid. Hence, whenever a current is produced, indicated even by the most delicate galvanometer, the metals employed are not perfectly homogeneous.

3. Metals become heterogeneous by the slightest alteration on their surfaces, either by mechanical or chemical agency; and the latter may be so slight as not to be ascertainable by any chemical test whatever. Even the catalytique forces of Berzelius may be sufficient to produce heterogenerity, indicatable by a galvanometer, on the surface of metals otherwise perfectly homogeneous.

Keeping in view these facts, it will be easily understood that two pieces of metal of the same kind, although perfectly homogeneous, can no longer be so after immersion in two different kinds of liquid. Hence it is, that the platinum which is placed in the nitric acid, when voltaically considered, is a different metal to the platinum placed in the alkaline solution. This superficial change, which is often accomplished in an infinitely short period of time, sometimes augments or raises the natural electromotive relation of metals, and sometimes diminishes, or even completely destroys it. The current produced by contact is generally, perhaps necessarily, accompanied by a chemical decomposition,t the results of which exercise a certain influence on the surfaces of the metals. This influence, I say, has always a tendency to neutralise, by compensation, the electromotive difference of the metals, to enfeeble the current, and, in time, to completely annihilate the action of the pile. By the batteries with separate compartments for each metal, a most excellent discovery of modern date, we can sustain or prolong the current at a constant rate of force, by preventing the liberation of the noxious bodies; an effect which cannot be obtained by the ordinary piles, in which one liquid only is employed.

4. A certain relation between the conductibility and the chemical condition of liquids is generally observed, although no decicive opinion on this point can as yet be formed.

This acknowledgment is sadly at variance with the previous conclusions of Jacobi, drawn from the experiments of Fechner!!!_Edit.

+ It is rather singular that our author, who we should suppose to be well acquainted with the simple plates of Volta, could arrive at such a conclusion. A moment's contact produces a momentary electric current. And by the dry electric piles, currents of twenty years, nay, of hundreds of years duration, may be produced, and still no chemical action, within the pile, taking place.--Edit.

« ZurückWeiter »