ELECTRICITY FROM HEAT. The de riving of electricity directly from the application of heat is interesting, though it has not proven of commercial value. Two different metals in contact usually show a difference of potential. This difference is most marked in the case of bismuth and antimony. When bars of these metals are soldered together at one end and the opposite ends connected by a copper wire, and a flame is applied to the point of junction a slight electric current is set up, flowing through the closed circuit thus formed. The cooling of the point of junction also sets up a current. An apparatus made on this principle is called a thermo-electric couple, and a series of such couples, arranged to work together, is called a thermopile; a still larger aggregation of thermo electric couples arranged in rings superimposed has been styled a thermo-electric generator. With any of these contrivances the current ob tained is so minute as to serve no purpose except experiment. The thermopile is valued
for experimental purposes because of the great constancy obtainable with a very slight current. Iron is not a good metal to use in a thermopile, because at certain temperatures its potential coincides with nearly all other metals, so that there would be no current when that tempera ture was reached.
Pyroelectricity is not to be confused with thermoelectricity because of the similarity of name. It treats of the phenomena of electric polarity in minerals on being heated or cooled. The quality of pyroelectricity is best shown in tournaline, a crystal of which on being heated from about 10° to 150° C. displays positive elec trification at one end and negative at the other; but on cooling the polarity is reversed and the positive and negative ends change places. Twin crystals of quartz also show the phenomena and other crystals in a lesser degree.