The space both outside and inside the substance of the conductor, but more especially the former, becomes a “magnetic field” in which delicately pivoted or suspended magnetic needles will take up definite positions and magnetic materials will become magnetized.
3. The Chemical effect;—
If the conductor be a liquid which is a chemical compound of a certain class called electrolytes, the liquid will be decomposed at the places where the current enters and leaves it.
Thermal Effect.—If a quantity of electricity were set flowing in a closed circuit and the latter offered no resistance, it would flow forever, just as a wagon set rolling along a circular railway would never stop if there were no friction.
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.