CHAPTER XXIV.

THERMOELECTRICITY.

=EXPERIMENT 174. To find whether electricity can be produced by heat.=

_Apparatus._ The home-made thermopile described in §421; astatic galvanoscope; connecting wires; candle or alcohol lamp.

=421. Home-made Thermopile.= (Fig. 134.) For this you need 3 hairpins, copper wire, a piece of wood about 3 in. long and 1 in. square on the ends, 2 pieces of tin, and some small nails.

Straighten the hairpins and scrape the coating off with sandpaper or a file. Scrape the insulation from 4 pieces of copper wire, each about 8 in. long. Twist the ends of the copper wire about the ends of the hairpins (Fig. 134), and then fasten the hairpins to the block. They may be held firmly by small nails which should be driven partly into the block and bent over. The hairpins at the right-hand side of the Fig. are shown to be near but not touching each other. This allows all to be heated at the same time.

The tin binding-posts may be nailed or screwed to the block, and if the bare copper wires 1 and 4 be placed under X and Y before they are screwed down they will be electrically connected. The ends of 1 and 4 may be held under the screw-heads. The block may be supported upon other blocks to raise it to the proper height, which will depend upon the length of the candle.

A thermopile in the form of a circle with several pairs of metals, can easily be made by fastening the hairpins to a piece of cardboard (Fig. 135) with a hole at the center. This may be supported by blocks, the heat being applied under the center.

=422. Directions.= (A) Arrange the apparatus as in Fig. 134. See that the astatic needle is properly adjusted, no magnets being near it.

(B) Heat the joints as shown, and watch the needle. Can a current be produced by heat?

(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.")