_=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. Several questions now come up. Can a _current_ of electricity in a conductor induce a _current_ in another conductor not in any way connected with the first? Can current electricity produce effects through space? Is there an electromagnetic induction? It has been seen that a current-carrying wire has a magnetic field, and that magnetic fields can act through space. It is evident, then, that a conductor will be surrounded and cut by lines of force when it is placed in a magnetic field, or near a wire or coil through which a current passes. Let us study this by experiments. =EXPERIMENTS 175-182. To study induced currents.= _Apparatus._ The two coils of wire (Nos. 89, 90); two short, soft iron cores (Nos. 92, 93); long iron core (No. 96); bar magnet (No. 97); astatic galvanoscope (No. 59); dry cell (No. 51); key (No. 55); horseshoe magnet; connecting wires with spring connectors (No. 54) on the ends (§ 226-230); coil of wire (No. 98) wound on an iron core; compass. [Illustration: Fig. 136.] =EXPERIMENT 175. To find whether a current can be generated with a bar magnet and a hollowed coil of wire.= =427. Directions.= (A) Arrange as in Fig. 136. The coil (No. 90) of fine wire is joined to A G (No. 59) as shown. Small pieces of tin or copper, 1 and 2, are used to make connections between the coil ends and wires, 3 and 4, which are attached to the galvanoscope. It is best to use the wires, 3 and 4, so that the coil will be 2 feet at least, from A G; otherwise the needle of A G might be affected by the magnet, M (No. 97).