As soon as the thermo-electrical current was dis covered, it was obvious that a compound thermo electrical circuit might be formed in analogy with Volta's complex hydro-electrical circuit. This consequence did not escape Dr. Seebeck, but dis covering some opposing circumstances, which we shall soon mention, he bestowed little labour upon this subject, to which he perhaps proposed to re turn another time. Baron Fourier and Professor Oersted undertook, without knowing this observa tion of Dr. Seebeck's, a similar research. Their first complex thermo-electrical circuit was a hexa gon formed of three pieces of bismuth and three of antimony soldered together. One of the sides was put in the magnetic direction, and a compass placed below it, when first one of the junctions was heat ed, then two, not adjacent junctions, were heated, at last three, still leaving between two heated junc tions one which was not heated. The compass needle changed its direction some degrees by the heating of one of 'the junctions, still more by the heating of two, and most when all the three junctions were heated. By cooling the three junctions by means of ice, and leaving the three others to the temperature of the atmosphere, similar and even more comparable effects were produced. By heat ing three alternating junctions, and cooling the other with ice, the effect rose to 60° of the compass used in the experiment. In another series of ex periments a rectangular circuit of 22 bars of anti mony and 22 of bismuth soldered together was em ployed. Here likewise as in the preceding experi ment, the combined effect of heating and cooling was employed. Now the circuit was opened by dissolving one of the junctions, and, in order to establish the circuit, when required, a little cup of brass destined to contain mercury, was soldered to each of the two bars, whose conjunction was inter rupted. A copper wire of about 4 inches in length, and inch in diameter re-established nearly the current; and by two parallel pieces of this wire the current was brought to the full effect. A wire of the same diameter, but a little more than three feet long, was found a tolerably good conductor, while a platina wire of inch and about 16 inches long scarcely transmitted a fortieth of the effect. Liquid acids and solutions of alkalies or other me tallic oxides, which prove excellent conductors in the hydroelectrical current, were found quite iso lating in the thermo-electrical circuit. Two discs of silver, separated only by a lamina of the thinnest blotting paper, moistened with sulphate of copper, isolated likewise the whole effect of the thermo electrical current.
The thermo-electrical current, even the most in tense that was tried, produced no visible chemical effect; nor was it capable of producing heat in thin metallic wires, probably because they are too feeble conductors of thermoelectricity.
The thermo-electrical circuit also produces no effect upon the electrical condensation.
It is very remarkable that, notwithstanding all that has been mentioned, the thermo-electric cir cuit makes a prepared frog palpitate, like the hy dro-electrical circuit. The communication between the extremities of the circuit and the nerves of the frog were made by means of plating wire, in order to guard against the influence of unequally oxidated surfaces.
Among circuits differing only by their length, the shortest has the greatest effect. A circuit of double length has not much more than half the ef fect. Complex circuits do not seem, therefore, at first sight, more efficacious than simple ones; the length being as much increased by the increased number of elements, as the effect should be height ened by the greater number of acting junctions; but comparing circuits of equal length whereof one has only two junctions, the other more, we see the true influence of the increase of acting junctions.
Plate DXXIII. Fig. 13 represents a simple circuit of antimony aa, and bismuth bb, where only one of the junctions is to be heated or cooled. Fig. 14 represents a complex circuit of the same length, formed of two pieces aa of antimony, and two pieces bb of bismuth. Two of the junctions of the latter arrangement, situated on the extremities of one diagonal are here heated or cooled. Under the same changes of temperature, where the circuit, Fig. 13, made the needle to deviate about 22 de grees, that of Fig. 14, made it to deviate about 30 degrees. Fig. 15 and 16 represent two circuits of double the extent of the former, one simple, one having three alternations. By the same differences of temperature, by which the arrangement, Fig. 15, gave from 13 to 15 degrees, that of Fig. 16 gave nearly 32 degrees.
In several complex circuits, it is found that the heating or cooling of one junction only produces twice the angular deviations of that added by the addition of each active junction more. The effect of one active junction, when the others are at rest, is by experiment found to be twice the effect of all the arrangements, divided by the sum of the elements + one. The effect of each addition of a new active junction is only half this quantity, and seems even to be in a decreasing ratio, when the number of junctions is great.
The effect of thermo-electricity upon the multi plier is very instructive. Fig. 17 represents an arrangement formed by two pieces b, b, of bismuth, and one piece a of antimony. When the two free extremities of b, b, are put in communication with the extremities of the wire of the multiplier, and one of the junctions between a and b is heated or cooled, the needle of the multiplier is deviated, but very little; when one of the junctions is only cool ed with ice, the effect is not so great as that of a disc of copper with one of silver, having common water as the liquid conductor. But when the ex tremities of b, b, are put in communication by means of a short piece of metal, the effect on the compass needle is considerable, whereas the effect of the hydro-electrical current of silver and copper, and even of silver and zinc,with common water as the liquid conductor, is scarcely sensible upon the same compass needle. This is a strong additional proof of the difficult transmission of thermo-electricity.
From all these observations we must conclude that the thermo-electric current produces an enor mous quantity of electricity, but in a state of ex ceedingly small intensity. In order to conceive this well, it is to be remarked that the intensity of electricity is measured by the attractions and re pulsions, whose force is in the inverse ratio of the squares of the distances,and that the quantity of elec tricity is measured by the number of equal surfaces which can be electrified by it to a certain degree of at traction and repulsion indicated by the electrometer. In the voltaic pile the intensity increases with the number of dies, the quantity with the surface of each of the discs. The greater the intensity the greater is the power of surmounting obstacles, or of pene trating through imperfect conductors; on the con trary, the greater the quantity the more perfect conductor is required to transmit it. The electri city produced by some thousand pairs of discs is able to penetrate a little lamina of air; that of some hundred pairs can at least penetrate through a con siderable length of water; that of two pairs cannot easily be transmitted but by the solid conductors and some of the powerful liquid conductors.