When the junction, of 2 different metals is heated, an electric current is generated, the greatest effect being obtained with antimony and bismuth. Such generators are known as thermo-electric piles. By multiplying them, increased force is produced; 60 well-constructed couples give a current equal to that from a 1-gal. Bunsen battery, and less than 3000 of Faure's couples, with an expenditure of 80 cub. ft. of gas per hour for heating, are equivalent to 50 Bunsen's. An improved form has recently been introduced by Sudre. One of its main features is the maintenance of the necessary difference of temperature between the two solder-ings of each couple by placing the couples between surfaces from which they are electrically insulated. An isolated thermo-electric couple is ordinarily composed of a metallic prism casting, and a polar plate of iron, copper, or German silver, soldered to each of its extremities. The plates do not interfere with the electric force obtained, and it is the bar (such as that of antimony and zinc) which produces the effect. When using 2 metals whose effects are combined, and which are easily fusible, e.g. antimony and bismuth, the couple is formed of 2 bars, joined by a cross bar soldered to each.

The total resistance of a couple is composed (1) of the resistance of the connecting plate; (2) of the resistance of the bar; (3) of a particular resistance at the points of contact or soldering between the plates and the bar. The plates should be of a sufficiently conductive metal, large and thick enough to present but feeble resistance, and as short as possible. The bar should have very little resistance under a small volume. Sudre takes as a datum the formula R=k L/S, in which k is a specific co efficient for the metal employed, L the length of the bar, and s its section. As the resistance depends on the ratio the volume of the couple may be diminished by lessening the length and sectional area in equal degrees, in which case the resistance will not be affected. The length given to the bar depends upon the difference of temperatures employed. For differences between 10° and 120° C. (50° and 248° F.), Sudre gives the couples a length of § in., whilst if the higher temperature reaches 300° C. (572° F.), the length varies from 3/4 to 1 1/8 in. The resistance at the points of contact or soldering is of the highest importance. The junction should be made so that the plate is in contact with the whole section of the bar.

The plate should penetrate to a very little depth within the bar, so as not to diminish the electro-motive force of the couple; for the effective difference of temperature is that of the 2 solderings, and this diminishes as the plates penetrate into the bar, and thus approach one another.