-483]

Thermoelectric Currents.

509 (fig. 346), the other pole is attached to the upper conductor by a wire, d. The latter is insulated in a large glass tube, r, filled with shellac, which is run in while in a state of fusion. Between the two conductors is the glass to be perforated, V. When this pre

sents too great a resistance, there is danger lest the spark pass in the coil itself, perforating the insulating layer which separates the wire, and then the coil is destroyed. To avoid this, two wires, e and c, connect the poles of the coil with two metallic rods, whose distance from each other can be regulated. If then the spark cannot penetrate through the glass, it bursts across with a bright spark and a loud report, and the coil is not injured.

CHAPTER XIII.

THERMOELECTRIC CURRENTS.

483. Thermoelectricity. In 1821, Professor Seebeck, in Berlin, found that by heating one of the junctions of a metallic circuit, consisting of two metals soldered together, an electric current was produced. This phenomenon may be shown by means of the apparatus represented in fig. 401, which consists of a plate of copper, mn, the ends of which are bent and soldered to a plate of bismuth, op. In the interior of the circuit is a magnetic needle

moving on a pivot. When the apparatus is placed in the magnetic meridian, and one of the solderings gently heated, as shown in the figure, the needle is deflected in a manner which indicates the passage of a current from n to m, that is, from the heated to the cool junction in the copper. If, instead of heating the junction, n

Fig. 401.

it is cooled by ice, or by placing upon it cotion wool moistened with ether, the other junction remaining at the ordinary temperature, a current is produced, but in the opposite direction; that is to say, from m to n. In both cases the current is more energetic in proportion as the difference in temperature of the solderings is greater.

Seebeck gives the name thermoelectric to this current, and the couple which produces it, to distinguish it from the hydroelectric or ordinary voltaic current and couple.

484. Thermoelectric series. If small bars of two different metals are soldered together at one end while the free ends are connected with the wires of a galvanometer, and if now the point of junction of the two metals be heated, a current is produced, the direction of which is indicated by the deflection of the needle of the galvanometer. By experimenting in this way with different metals, they may be formed in a list such that each metal gives rise to positive electricity when associated with one of the following, and negative electricity with one of those that precede; that is, that in heating the soldering, the positive current goes from the positive to the negative metal across the soldering. just as if the soldering represented the liquid in a hydroelectrical element; hence out of

-485]

Nobili's Thermoelectric Pile.

511 the element, in the connecting wire in the galvanometer for instance, the current goes from the negative to the positive metal. Thus a couple, bismuth-antimony, heated at the junction would correspond to a couple, zinc-copper, immersed in sulphuric acid.

Of all bodies, bismuth and selenium produce the greatest electromotive force; but from the expense of this latter element, and on account of its low conducting power, antimony is generally substituted. The antimony is the negative metal but the positive pole, and the bismuth the positive metal but the negative pole, and the current goes from bismuth to antimony across the junction.

485. Nobili's thermoelectric pile.-Nobili devised a form of thermoelectric battery, or pile as it is usually termed, in which there are a large number of elements in a very small space. For this purpose he joined the couples of bismuth and antimony in such a manner, that after having formed a series of five couples, as represented in fig. 403, the bismuth from 6 was soldered to the antimony of a second series arranged similarly; the last bismuth of this to the antimony of a third, and so on for four vertical series, containing together twenty couples, commencing by antimony, finishing by bismuth. Thus arranged, the couples are insulated from one another by means of small paper bands covered with varnish, and then enclosed in a copper frame, P (fig. 402), so that only the solderings appear at the two ends of the pile. Two small copper binding screws, m and n, insulated in an ivory ring, communicate in the interior, one with the first antimony, representing the positive pole and the other with the last bismuth, representing the negative pole. These binding screws communicate with the extremities of a galvanometer wire, when the thermoelectric current is to be observed.

Fig. 402.

Fig. 403.

A Nobili's pile in combination with a galvanometer constitutes the most delicate and accurate means of measuring a temperature. Such an arrangement was first used by Melloni in his researches on the transmission of radiant heat. The arrangements he used is represented in figure 404.

On a wooden base, provided with levelling screws, a graduated copper rule, about a yard long, is fixed edgeways. On this rule the various parts composing the apparatus are placed, and their distances can be fixed by means of binding screws. a is a support for a Locatelli's lamp, or other source of heat; F and E are screens; C is a support for the bodies experimented on, and m is a thermo

electrical battery. Near the apparatus is a galvanometer, D, which has only a comparatively few turns of a tolerably thick (1 mm.) copper wire. Such galvanometers are called thermomultipliers. The delicacy of this apparatus is so great that the heat of the hand is enough, at a distance of a yard from the pile, to deflect the needle of the galvanometer,