2IO ELECTROMOTIVE FORCE. chap.
electric energy can (theoretically) be totally converted into mechanical work. (Practically, of course, the energy trans- formation cannot be carried out without loss of work, because no machine works ideally; the best electric motors give a yield of about 95 per cent, in mechanical work.) The free energy of an element amounts therefore to 23,070 . P for every gram-equivalent decomposed, when P is the electromotive force of the element in volts. If W is the corresponding total energy, measured by the heat change, the difference, W - 23,070 . Py is the houiid energy.
The free enei^y of a system plays a very important part ; it gives, so to say, the maximum work which the system^ is capable of doing when a certain change takes place. The complete using up of the free energy is only conceivable in the case of reversible processes ; and in this connection it must be noticed that in reality any process can only be carried out more or less approximately in a reversible manner ; part of the free energy is always lost in over- coming unavoidable friction resistances. The free energy of a system always decreases when a spontaneous process takes place in it. It corresponds with the amount of work stored in the system. Thermochemists formerly believed (erroneously) that this store of work was represented by the total heat, in which case Thomson's rule would be quite valid.
The rule has a certain practical importance, for it may be applied in estimating the electromotive force of a new galvanic combination, the corresponding heat change of the reaction being generally known from direct measurements.
G. Meyer's Concentration Element.— The work which can be obtained by the decomposition of a gram-equivalent in an element is given by 23,070. P. Occasionally this work A can be measured in another way. Then from the equation —
A = 23,070 . P the electromotive force of the element can be calculated.
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