To Editors of the Philosophical Magazine and Journal ¹

GENTLEMEN,

IN an abstract of Professor William Thomson’s Mechanical Theory of Thermo-electric Currents, given in your Supplementary Number for July, reference is made to the well-known experiment of Peltier on the absorption of heat at a bismuth and antimony joint. This has drawn from Mr. Adie a brief communication, published in your Number for September, from which is appears that the writer has never been able to obtain Peltier’s result; he virtually denies its existence, and affirms the true state of the case to be that less heat is developed at some junctions than at others, but that cold is never generated. An objection precisely similar to that now urged by Mr. Adie induced Lenz to repeat the experiment fifteen years ago.² To the experiment of Lenz I took the liberty of drawing Mr. Adie’s attention in your October Number; I did so because Mr. Adie had never mentioned it in his remarks, and it seemed to me to offer a proof of the absorption of heat so obvious as to be immediately appreciated. It does not however appear so to Mr. Adie, for in your last Number I find that he suggests a hygrometric action as the probable cause of the diminution of temperature observed by Lenz. I shall ill occupy your space were I to dwell upon conjectures where the ‘law and testimony’ of experiment are so near at hand, and fact so readily attainable. If the following results do not convince Mr. Adie, they will perhaps be the means of clearing away whatever doubt his remarks may have created in the minds of others.

Experiment No. 1.-In Plate IV.³ Fig. 1, A is a bar of antimony, B a bar of bismuth, both bars being brought into close contact at J. To the free ends of the bars the wires w w′ are soldered, and dip into the little pools of mercury m m′; c is a piece of cork through which the wires pass, and by taking which in the fingers the wires w w′ may be easily moved from the pools m m′ to m m″, the warming of the wires being prevented by the cork. From m m″ wires proceed to a galvanometer, G, whose needles prove themselves to be perfectly astatic by setting at right angles to the magnetic meridian.⁴ B is a single cell of Bunsen, from which, when matters stand as in the figure, a current can be sent through the bismuth and antimony pair.

The voltaic circuit having been established, the current-a very feeble one-was permitted to circulate for two minutes, its direction being from antimony to bismuth across the junction ; at the end of the time specified the wires w w′ were moved from m m′ to m m″, a thermo-circuit being thus formed in which the galvanometer was included ; the index of the instrument was at once deflected, and the extreme limit of its first impulsion was noted ; it amount to

75°.

The deflection in this case was similar in direction to that produced when the warm finger was placed upon the junction.

The wires w w′ were moved back to their former position, and the apparatus was suffered to cool; by crossing the wires b b′, causing the former to dip into m and the latter into m′, the voltaic current was reserved, its direction across the junction being now from bismuth to antimony; the same time of circulation being allowed, on establishing the thermo-circuit, as before, a deflection of

68°

was observed. The deflection was the same as that produced when a small glass containing a freezing mixture was placed upon the junction.

But Mr. Adie will probably urge, that it is not the cold developed at J, but the heat developed at some of the other points, which caused the deflection here. I will not pause to discuss the objection, but will proceed to an experiment which deprives it of all force.

Experiment No. 2.-AA′ is a bar of antimony, BB′ is a bar of bismuth east as in fig. 2, and in contact at the centre. From the cell B a current was sent through the system, and during its circulation the ends g g′ were unconnected; neither heating nor cooling of these ends by the current was therefore possible. The direction of the current across the junction was first from antimony to bismuth. After a short period of circulation the current was interrupted, and the ends of the wires w w′ were dipped into the mercury cups g g′, which were also in contact with A′B′; the index was driven through an arc of

40°.

The sense of the deflection in this case showed that the junction had been heated.

The current was reversed, its direction across the junction being now from bismuth to antimony; proceeding as before, the deflection was

30°.

The sense of this deflection was the same as that produced when the temperature of the junction was lowered by a freezing mixture.

I see no escape here from the conclusion that heat has been absorbed; for the ends g g′, exposed as they are to the atmosphere, must have its temperature, while the ends m m′, on which suspicion might reasonably rest, the current having passed through them, are wholly excluded from the thermo-circuit. The reader will observe that this is merely a modification of Lenz’s experiment with the metallic cross.

But Mr. Adie has tried the cross, and it does not satisfy him; very well, we will discard it, and proceed at once to an experimentum crucis. If the arms A′ B′ are not actually included in the voltaic circuit, they may seem to be in suspicious connexion with it. We must remove this source of doubt.

Experiment No. 3.-A and B, fig. 3, represent, as before, the bismuth and antimony couple, united at one end. M is a small chamber, hollowed out in a piece of cork and filled with mercury. A′ B′ is a second delicate thermo-electric pair, connected with the galvanometer, but wholly unconnected with A B. The wires w w’ are sufficiently strong to support A′ B′, so that the junction stands vertically over M, a slight pressure being sufficient to cause the wedge-shaped end of the pair to descend into the chamber of mercury. The whole arrangement was permitted to remain in a room until the temperature of the surrounding atmosphere was attained. Matters being in this state, when the pair A′ B′, which I will call the test-pair, was dipped into the mercury M, no effect was produced on the galvanometer. Now the mercury must partake of the changes of temperature of the junction with which it is in contact, and the nature of these changes will be ascertained with great precision by examining the mercury at proper intervals by means of the test-pair.

The voltaic circuit was closed, and the current allowed to circulate for three minutes, passing in the first place from bismuth to antimony. The current was then interrupted, and the test-pair was immediately dipped into the pool of mercury; the index of the galvanometer was driven through an arc of

40°.

The deflection was similar to that produced by immersing the end of the test-pair in a freezing mixture. Hence in this case heat was undoubtedly abstracted from the mercury during the passage of the current.

The apparatus being permitted to resume its equilibrium, the voltaic current was caused to traverse AB in an opposite direction. At the end of three minutes the test-pair was again immersed, and a deflection of

45°

was the consequence. The deflection was opposed to the former one, and demonstrated the generation of heat at the junction.

I am at present unable to see what possible objection can be brought against this last experiment. A hygrometric effect is out of the question; and the test-pair A′ B′ being wholly unconnected with the voltaic current, cannot in any way be influenced by the latter. The results observed are evidently pure effects of the heating and cooling of the junction.

It will perhaps be permitted me to cite a single additional experiment, which exhibits all the necessary evidence without the reversion of the voltaic current.

Experiment No. 4.-B, fig. 4, is a curved bar of bismuth, with each end of which a bar of antimony, A, is brought into close contact. In front of the two junctions are chambers, hollowed out in cork and filled with mercury as before. A current was sent from the cell B in the direction indicated by the arrow; at M it massed from antimony to bismuth, and at M′ from bismuth to antimony. Now if Peltier’s observation be correct, we ought to have the mercury at M warmed, and that at M′ cooled by the passage of the current. After three minutes’ circulation the voltaic circuit was broken, and the test-pair dipped into M; the consequent deflection was

38°,

and the sense of the deflection proved that at M′ head had been absorbed.

The needles were brought quickly to rest at zero, and the test-pair was dipped into M; the consequent deflection was

60°;

the sense of the deflection proved that a M heat had been generated.

The system of bars represented in fig. 4, being imbedded in wood, the junction at M cooled slowly, and would have taken a quarter of an hour at least to assume the temperature of the atmosphere. The voltaic current was reversed, and three minutes’ action not only absorbed all the head at M, but generated cold sufficient to drive the needle through an arc of 20° on the negative side of zero.

These experiments, Gentlemen, corroborate a result which to my mind is sufficiently well established without them. Nevertheless I would say, that the conclusions of Mr. Adie are such as a restricted examination of the subject will most probably lead to. I have no doubt as to the correctness of his results described in the September Number of the Magazine; but I have just as little doubt, that had Mr. Adie varied the strength of his current sufficiently, he would have spared himself the statement, that ‘in his experiments he had never met a fact which in the least encourages the view that electricity reduces temperatures’.

I remain, Gentlemen, | Your obedient Servant, | JOHN TYNDALL.

Queenwood College, | November 1852.

From the published letter, Article LXVI. ‘On the Reduction of Temperatures by Electricity [with a plate]’, Phil. Mag., 4:27 (1852), pp. 419–23.