Lessons in elementary physics / by Balfour Stewart.

Balfour Stewart

Date:: 1895

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

420/502 page 400

$tact with this copper will be + 3. In like manner the state of the highest zinc plate will be + 5. Going downwards now from the middle copper plate, this will communicate its charge (— i) by means of the cloth to the zinc below it, of which the charge will therefore be - 1. But the copper in contact with it will be two units more negative on account of heterogeneity, and will therefore be — 3 ; while in like manner the lowest copper will be —5. Thus for five pairs there will be an electrical difference between the top and bottom of ten, or five times as much as that given by a single pair. Nevertheless Volta appears to have erred, in his ignorance of the laws of energy, by supposing that the mere contact of heterogeneous metals could account for the large amount of energy exhibited by a voltaic battery when its extremities are connected together ; for then it generates heat, and is capable of performing mechanical work of various kinds. It is, in fact, a powerful kind of energy, and this electricitv in motion has been described by us in Art. 108 as one of the varieties of molecular energy. Indeed, it is very manifest that since the tendency of electricity is to equalise itself, in order to procure a continuous stream of electricity we must have a reservoir which is always giving it out, just as much as in order to obtain a continuous flow of water we must have a reservoir of some kind containing water. Now the mere contact of heterogeneous metals cannot supply us with a constant stream of electricity, for if it could we should at once have a kind of perpetual motion, which is manifestly impossible. V e must, therefore, look somewhere else for the source of the energy of the voltaic circuit. It was seen by those who came after \’olta, that in order to produce a continuous current of electricitj' a quantity of the oxidisable metal of the circuit must, so to speak, be burned, being converted into a metallic salt ; and they imagined that the electrical separation of the voltaic pile was due to this chemical action, and not to the mere contact of heterogeneous metals. 382. Volta’s Error.—But Sir W. Thomson (now Lord Kelvin) has made an experiment, which .appears to show that Volta was right in supposing the electrical separation to be$

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