CHAPTER X
THE ENERGY THEORY OF ELECTROLYSIS—ELECTRICAL UNITS—ELECTRICAL OSCILLATIONS
Electrolysis and Electrical Units
In December 1851 Thomson communicated an important paper to the Philosophical Magazine on "The Mechanical Theory of Electrolysis," and "Applications of Mechanical Effect to the Measurement of Electromotive Forces, and of Galvanic Resistances, in Absolute Units."
In the first of these he supposed a machine of the kind imagined by Faraday, consisting of a metal disk, rotating uniformly with its plane at right angles to the lines of force of a uniform magnetic field, and touched at its centre and its circumference by fixed wires, to send a current through an electrochemical apparatus, to which the wires are connected. A certain amount of work W was supposed to be spent in a given time, during which a quantity of heat H was evolved in the circuit, and a certain amount of work M spent in the chemical apparatus in effecting chemical change. If H be taken in dynamical units, W = H + M.
The work done in driving the disk, if the intensity of the field is I, the current produced c, the radius of the disc r, and the angular velocity of turning w, is ½Ir2cw.
Thomson assumed that the work done in the electrochemical apparatus was equal to the heat of chemical combination of the substance or substances which underwent the chemical action, taken with the proper sign according to the change, if more compound substances than one were acted on. Hence M represented this resultant heat of combination.
The electrochemical apparatus was a voltameter containing a definite compound to be electrolysed, or a voltaic cell or battery. And by Faraday's experiments on electrolysis it was known that the amount of chemical action was proportional to the whole quantity of electricity passed through the cell in a given time, so that the rate at which energy was being spent in the cell was at any instant proportional to the current at that instant.
The chemical change could be measured by considering only one of the elements set free, or made to combine, by the passage of the current, and considering the quantity of heat θ, say, for the whole chemical change in the cell corresponding to the action on unit mass of that element. Thus if E denote the whole quantity of that element operated on the heat of combination in the vessel was θE. If E be taken for unit of time, and ε denote the quantity set free by the passage of unit quantity of electricity, then E = εc, since a current conveys c units of electricity in one second. The number ε is a definite quantity of the element, and is called its electrochemical equivalent. Again, from Joule's experiments, H = Rc2, if R denote the resistance of the current, and so
