Thermal Currents with two Metals.—A current is always obtained when the point of junction of any two metals is heated. The two metals which show this property in the greatest degree are bismuth and antimony.
When a bar of antimony, A (fig. 1), is soldered to a bar of bismuth, B, and their free extremities are connected with a galvanometer, G, on the junction being heated, a current passes from the bismuth to the antimony, as shown in the figure. When S is chilled by applying ice, or otherwise, a current is also produced, but in the opposite direction. Such a com bination constitutes a thermo-electric pair. Applying the same mode of explanation to this pair that we apply to the galvanic pair (see GALVAN ism), bismuth is positive within and negative without the pair, antimony negative within and positive without the pair. Bismuth thus forms the negative pole, but positive element; antimony the positive pole, but neg ative element of the pair. The metals may be classed in thermo-electric just as in trochemical order. The following table gives them in this order, the direction of the arrow showing how the current goes within the pair. The order and numbers in this table, which are for temperatures between 40' and 100° Fahr., are those given by Dr. Ma thiessen. For other temperatures, the table would be different for several of the metals.
It will be seen, that metals like bismuth and antimony, which have a crystalline struc ture, are best suitedfor a thermo-electric pair.
Tourmaline, when heated, shows an opposite electricity at each end. If it had a low con ducting power like the metals just named, we might expect from it a thermo-electric current instead of mere polarity. It is probable that the crystalline structure, however, accounts for the appearance of electricity in both cases.
Thermo-electric Pile or bismuth-antimony pair is of very little power. To increase this, several pairs are associated tog,ether, and the same tension-arrangement is adopted as in a galvanic battery. The heat in this case must be applied only to one row of soldered faces. The current depends on the difference of temperature of the two sides. When a strong current is required, the one series must be kept in ice, or in a freezing mixture, while the other is exposed to heat radiating from a red-hot plate of iron. As in the galvanic pair, the electro-motive force is proportionate to the number of pairs; the size of the bars, like the size of the galvanic plates, merely aiding to diminish the resistance. The electro-motive force of a thermo-electric battery is small; to Dr. Mathiessen, that of 25 bismuth-tellurium pairs equals one cell of Dan iell's battery, when the one series is kept at 32° Fahr. and the other at 212° Fahr. In con
sequence of the low electro-motive force of the thermo-electric battery, the galvanometer to be used with it must introduce as little resistance as is consistent with the best effect on the needle. Hence special galvanometers are used, in which the coil wire is short (200 turns) and thick inch); these are called thermo-galvanometers.
When a great number of pairs are formed into a bat tery, they may be conveniently arranged as in fig. 2, which shows cne of 30 pairs. The odd faces, 1, 3, 5, etc., are exposed on the one side, and the even faces, 2, 4, 6, etc., on the other. The terminal bars are connected with the binding screws n, p. The interstices of the bars are filled with insulating matter (gypsum) to keep them separate, and the frame in which the whole is placed is of non-conduct ing matter. Such a pile in conjunction with a thermo galvanometer (see GALVAxTSM) forms the most delicate thermometer for radiant heat, and is generally called a therrno-multiplier. When placed in a room, the temperature of which is equable all round, no current is produced, bt..t if heat be radiated more on one side than another, a current ensues. If the hand, for instance, be brought near on the one side, a current indicates its radiant power; or if a piece of ice be brought near, a current is also shown, but moving in the opposite way.
Thermal Effects produced by the Galvanic heat or cold produces a cur rent at the junction of two dissimilar conductors, we should expect that if a galvanic current be made to pass through the junction, heat or cold would follow, and such is found to be the fact. When a current from a voltaic cell passes through a system of three rods of bismuth, antimony, and bismuth, at the junction where the current passes from bismuth to antimony, cold is produced; and at the other, from antimony to bis muth, heat. If, for instance, water be placed in a hollow at either junction, cooled to 32° Fahr, it will become frozen when the current passes from the bismuth to the antimony. When the junction of these two metals is put into the bulb of an air thermometer, so that a current can be sent through it in either way, the air expands when the current goes from antimony to bismuth, but contracts when it goes in the opposite way. See THERMO-ELECTRICITY, where the theory of energy is applied to the explanation of the various phenomena.
Seebeck was the discoverer (1821) of thermo-electricity; Nobili invented the thermo electric pile (1834); Peltier (1834) first observed the thermal effects of galvanic currents at the junction of heterogeneous conductors.