By the installing of a storage plant many natural but small sources of power may be utilized in furnishing light and power; sources which otherwise are not available, because not large enough to supply maximum demands. The force of the tides, of small water powers from irrigating ditches, and even of the wind, come under this heading. [Illustration: Fig. 85.] As a regulator of pressure, in case of fluctuations in the load, the value of a storage plant is inestimable. These fluctuations of load are particularly noticeable in electric railway plants, where the demand is constantly rising and falling, sometimes jumping from almost nothing to the maximum, and _vice versa_, in a few seconds. If for no other reason than the prevention of severe strain on the engines and generators, caused by these fluctuations of demand, a storage plant will be valuable. CHAPTER X. HOW ELECTRICITY IS GENERATED BY HEAT. =88. Thermoelectricity= is the name given to electricity that is generated by heat. If a strip of iron, I, be connected between two strips of copper, C C, these being joined by a copper wire, C W, we shall have an arrangement that will generate a current when heated at either of the junctions between C and I. When it is heated at A the current will flow as shown by arrows, from C to I. If we heat at B, the current will flow in the opposite direction through the metals, although it will still go from C to I as before. Such currents are called _thermoelectric currents_. [Illustration: Fig. 86.] Different pairs of metals produce different results. Antimony and bismuth are generally used, because the greatest effect is produced by them. 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 bismuth to antimony across the joint. By cooling the juncture below the temperature of the rest of the circuit, a current will be produced in the opposite direction to the above. The energy of the current is kept up by the heat absorbed, just as it is kept up by chemical action in the voltaic cell. =89. Peltier Effect.= If an electric current be passed through pairs of metals, the parts at the junction become slightly warmer or cooler than before, depending upon the direction of the current. This action is really the reverse of that in which currents are produced by heat. [Illustration: Fig. 87.] =90. Thermopiles.= As the E.M.F. of the current produced by a single pair of metals is very small, several pairs are usually joined in series, so that the different currents will help each other by flowing in the same direction. Such combinations are called thermoelectric piles, or simply _thermopiles_.