(C) Remove the connector on wire 6 from Y to M, thus cutting one pair out of the circuit. Heat the joints again and compare the strength of the current with that produced in (B).
(D) See whether much current is produced by one pair. From results obtained do you see any relation between the strength of the current and the number of pairs?
423. Thermoelectricity is produced by heating the junction between two metals. Different pairs of metals produce different results. Antimony and bismuth are often used. If the end of a strip of bismuth be soldered to the end of a similar strip of antimony, and the free ends be connected to a galvanometer of low resistance, the presence of a current will be shown when the point of contact becomes hotter than the rest of the circuit. The current will flow from the bismuth to antimony across the joint. By cooling the junction below the temperature of the rest of the circuit a current will be produced in the opposite direction.
Thermoelectric currents have a low potential. The energy of the current is kept up by the heat absorbed.
424. Peltier Effect. The action noted in [§ 423] can be reversed; that is, if a current from a battery be sent through the metals, the parts at the junction become slightly warmer or cooler than before, depending upon the direction of the current. This is known as the Peltier Effect, the heat not being due to the resistance to the current.
425. Thermopiles. As the E. M. F. of the current produced by a single pair of metals is small, several pairs are usually joined in series in such a way that the different currents help each other and flow in the same direction. Such combinations, usually made of antimony and bismuth, are called thermoelectric piles, or simply thermopiles. They are useful in detecting very small differences in temperature. The heat of a match, or the cold of a piece of ice, will produce a current even at some distance, the thermopile being connected with a sensitive short-coil astatic galvanometer. (See "Things a Boy Should Know About Electricity.")
CHAPTER XXV.
INDUCED CURRENTS.
426. Electromagnetic Induction. You have seen, by experiments, that a magnet has the power to induce another piece of iron or steel to become a magnet. You have also seen, in the study of static electricity, that an electrified body has the power to act through space upon another conductor. A body may be polarized and charged with static electricity by induction.