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CHAPTER III. THE ELECTRICITY OF HEAT. In the year 1821 Professor Seebeck, of Berlin, discovered a third source of electricity. Volta had found that two dissimilar metals in contact will produce a current by chemical action, and Seebeck showed that heat might take the place of chemical action. Thus, if a bar of antimony A (fig. 22) and a bar of bismuth S are in contact at one end, and the junction is heated by a spirit lamp to a higher temperature than the rest of the bars, a difference in their electric state or potential will be set up, and if the other ends are joined by a wire W, a current will flow through the wire. The direction of the current, indicated by the arrow, is from the bismuth to the antimony across the joint, and from the antimony to the bismuth through the external wire. This combination, which is called a "thermo-electric couple," is clearly analogous to the voltaic couple, with heat in place of chemical affinity. The direction of the current within and without the couple shows that the bismuth is positive to the antimony. This property of generating a current of electricity by contact under the influence of heat is not confined to bismuth and antimony, or even to the metals, but is common to all dissimilar substances in their degree. In the following list of bodies each is positive to those beneath it, negative to those above it, and the further apart any two are in the scale the greater the effect. Thus bismuth and antimony give a much stronger current with the same heating than copper and iron. Bismuth and selenium produce the best result, but selenium is expensive and not easy to manipulate. Copper and German silver will make a cheap experimental couple:— POSITIVE Bismuth Cobalt Potassium Nickel Sodium Lead Tin Copper Platinum Silver Zinc Cadmium