[Illustration: FIG. 88.--The Seebeck effect: If in a complete metallic circuit having junctions of dissimilar metals, the junctions are at different temperatures, then generally a steady current will flow in the circuit as long as the differences of the temperatures of the junction is maintained. To demonstrate this, a piece of copper K bent in the shape seen in the figure, was placed on a block of bismuth A B, carrying a pivoted magnetic needle N S; as soon as the equality of temperatures was altered by either heating or cooling one of the junctions of the two metals, the needle indicated a current which continued to flow as long as the difference of temperature was maintained at the junctions. The movement of the needle indicated the direction in which the current flowed. If, for instance, the north junction B were heated, the N pole moved eastwards, showing that at the heated junction the current flows from the bismuth to the copper, at the cold junction from the copper to the bismuth.] When matter in motion is stopped by friction, the energy of its motion is converted into heat by the friction thus causing the matter to come to rest. Similarly, when electricity in motion, that is, an electric current is stopped by resistance, the energy of its flow is transformed into heat by the resistance of the circuit. If the terminals of a battery be joined by a short thick wire of low resistance, most of the heat will be developed in the battery, whereas, if a thin wire of high resistance be used it will become hot, while the battery itself will remain comparatively cool. To investigate the development of heat by a current, Joule and Lenz used instruments on the principle of fig. 87, in which a thin wire joined to two stout conductors is enclosed within a glass vessel containing alcohol, into which is placed a thermometer. The resistance of the wire being known, its relation to the other resistances can be calculated. Joule found that the number of heat units developed in a conductor is proportional to: 1. The resistance; 2. The square of the current strength; 3. The time that the current lasts. Joules’ law may be stated as follows: _The heat generated in a conductor by an electric current is proportional to the resistance of the conductor, the time during which the current flows, and the square of the strength of the current._ The quantity of heat in calories may be calculated by use of the equation, calories per second = volts × amperes × .24. (1) The total number of calories H developed in t seconds will be given by H = P.D. × C × t × .24. (2) EXAMPLE--If a current of 10 amperes flows in a wire whose terminals are at a potential difference of 12 volts, how much heat will be developed in 5 minutes? Substituting in equation (2): 10 × 12 × (60 × 5) × .24 = 8640 calories. Since by Ohm’s Law potential difference = I × R substituting IR for P.D. in (2)