Lessons in elementary physics / by Balfour Stewart.

  • Balfour Stewart
Date:
1895
Catalogue details

Licence: Public Domain Mark

Credit: Lessons in elementary physics / by Balfour Stewart. Source: Wellcome Collection.

    459/502 page 439
    its resistance in the same proportion, and it therefore fol- lows that by reducing to one half the cross section of a wire, we double the amount of heat generated in it by the passage of the same quantity of electricity. Again, since this double amount of heat has only half the amount of metal to influence, it follows that the initial rise of temperature will be increased fourfold ; that is to say, the initial increase of temperature pro- duced in one second by the passage of the same quantity of electricity will vary inversely as the square of the cross section. 429. Heat and Current.—In the next place, the heat gen- erated in a given time is proportional to the sqtiare of the strength op the ctirrent. We may deduce this from the previous law by supposing that we have two wires of single thickness close together, while single currents are made to pass through each simultaneously, so that we may imagine one current to go through the one wire and one through the other. Therefore by means of this double current going through a double wire, twice as much heat will be generated in a given time as by a single current going through a single wire : but we have just seen that when a double current goes through a single wire twice as much heat is generated as when it goes through a double wire ; that is to say, four times as much as when a single current. goes through a single wire. 430. Distribution of Heat in Circuit.—We have pre- viously seen (Art. 423) that the quantity of electricity which passes in unit of time through every cross section of a closed circuit is the same ; and we have also seen that the resistance is inversely proportional to the conductivity ; we can therefore, if we know the electric conductivity of the various materials of which the circuit is composed, find the distribution of heat in the various parts of the circuit. Thus let one part be composed of a metre of silver wire two square millimetres in cross section, and another of five metres of zinc wire four square millimetres in cross section, what will be the relative heating effects of the current on these two wires? If we call the heat produced in the first wire unity, that produced in the second will be in which I 2 X t; X 4 8-62, expression the second factor is on account of conduc-
    No text description is available for this image
    No text description is available for this image
    No text description is available for this image