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Page:Popular Science Monthly Volume 24.djvu/763

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vicinity of coils of wire, or coils of wire in the vicinity of magnets. The significance of his discovery was so apparent that inventors began at once to devise means for generating currents upon an extended scale, by moving large magnets in the vicinity of large coils of wire by means of machinery; and this mechanical system has now been brought to such perfection that the cost of producing a horse-power of electrical energy can be as easily and almost as accurately calculated as the cost of producing a horse-power in a steam-engine or any other familiar apparatus.

In order to arrive at a clear comprehension of the present state of the art, it will be necessary to remember that any work which we perform must be performed by the expenditure of a certain and absolute amount of energy, and that we can not create this energy, but can only obtain it by changing the form of some other kind of energy. In the voltaic battery, as we have said, the electrical energy is obtained by transforming the heat of the chemical action going on in the cell into electrical energy, so that the amount of the latter that can be got out of any voltaic battery is limited by the amount of energy of the chemical combination. Now, the metal ordinarily used for furnishing chemical energy in a voltaic battery is zinc, and the heat of combination of zinc with oxygen is only about one sixth of that of coal, while its cost is more than twenty times as great; so that, to get the same amount of energy from zinc as from coal, would cost about one hundred and twenty times as much. Now, in the mechanical method of generating electricity, the electrical energy is produced by the mechanical means of moving large magnets near coils of wire; but the mechanical energy necessary to do this is obtained by the combustion of coal (i.e., the chemical combination of coal with oxygen).

It would be incorrect, however, to say that we can in this way produce electricity one hundred and twenty times as cheaply as by a battery, because there is an enormous loss in converting the heat of combustion of the coal into electricity, whereas the voltaic battery produces the electricity directly. The losses in converting the energy of the combustion of coal into mechanical energy are so prodigious that even a theoretically perfect engine could not get hold of more than from twenty to twenty-five per cent of the total energy in the coal, on account of the loss of the heat; so that, if an engine (a good one) has an efficiency of eighty per cent, it can not actually convert into work as much as twenty per cent of the total energy in the coal. The loss now in converting this mechanical energy into the electrical energy in the circuit where it is desired may be taken as about fifteen per cent, so that only about from fifteen to seventeen per cent of the total energy of the burning coal may be looked for in the electrical circuit. But, as the original cost of the coal is only 1120 of that of the zinc furnishing an equal amount of energy, we see that the mechanical method