TRANSACTIONS OF SECTION B. 979 
In actual practice the work ultimately done by the passage of a current through 
an electrolyte is not solely chemical decomposition ; a disturbance of the thermal 
equilibrium also tales place, usually heat evolution. A certain amount of heat 
evolution, in fact, necessarily ensues, owing to the resistance proper of the medium. 
‘Tn accordance with ‘ Joule’s Law’ the work done in this form is C*Ré (where C is 
the current, R the resistance proper, and ¢ the time), which may be written e’g, 
since g = Cf, and by Ohm’s Law CR represents an E.M.F’. which may be written e’. 
Tf, then, a total fall of potential of E occur, part of this, e’,is due to heat evolution 
through resistance proper; the rest, H —e’, is due to other causes, viz. chemical 
decomposition modified by secondary actions taking place, either spontaneously in 
the products of electrolysis themselves, or by their causing chemical changes in the 
electrodes. These secondary actions are usually of such a nature as to cause as 
final result the production of more or less sensible heat ; but generally a greater or 
less fraction of the energy due to these acts in the same sense as the current itself; 
that is, the secondary actions assist the current, and cause a less amount of lower- 
ing of potential to take place than would otherwise be the case. That portion of 
the energy due to these secondary changes that thus assists the current may con- 
veniently be spoken of as adjuvant, and the rest as nonadjuvant. 
With certain kinds of electrolytes and electrodes, the amount of adjuvant 
energy is sufficient not merely to equal the lowering of potential due to the passage 
of the current per se, but to preponderate thereover, so that a negative lowering of 
potential results as the current passes. Such cells constitute ordinary electromotors 
or ‘ voltaic circles,’ and usually require no external current to start their action. 
Whether an electrolytic cell be one in which an actual lowering of potential 
takes place during the passage of a current from an external source, or one capable 
of causing an increment in potential of itself without any current passing derived 
ab extra, it is invariably found that the actual potential difference subsisting 
when a current passes, after correction for the quantity e’ due to heat evolution 
in consequence of resistance proper, is not a constant quantity, but varies with the 
degree of concentration of the solutions and the nature of the surfaces of the elec- 
trodes, and with the density of the current (ratio of current to electrode superficies). 
With a decomposing cell (where actual lowering of potential takes place) this 
corrected potential difference increases, ceteris paribus, with current density ; and 
with an electromotor (where negative lowering of potential ensues) it decreases 
therewith ; the same rule being observed throughout, that the greater the density 
the greater is the amount of nonadjuvant energy due to secondary actions. In other 
words, the ‘ counter E.M.F.’ of a decomposing cell, H—e’, continually increases 
with increasing current density, constantly tending towards a limiting value attain< 
able theoretically with an infinite current ; whilst the negative counter E.M.F. (or 
positive E.M.F.) of a voltaic cell or electromotor is at its maximum when the 
current density is extremely small, and continually decreases as the current density 
increases. 
A large number of experiments have been made (as yet unpublished) with a 
view of deducing for various electrolytes the limiting values of the ‘ counter 
E.M.F.s.’ set up during electrolysis, the method adopted being the construction of 
curves with current densities and counter E.M.F.s as co-ordinates from which 
empirical exponential equations might be deduced leading to the evaluation of 
the limiting values for infinite densities. Much labour is necessarily entailed in 
this class of experiment in order to obtain any results of value even as approxima- 
tions. The results obtained so far show that these limiting values far exceed those 
corresponding with the heat evolutions ensuing when the final products of the 
electrolytic decomposition are made to recombine so as to reproduce the compound 
experimented with. Thus, for example, the heat disengaged when one gramme of 
hydrogen and eight of oxygen (both gaseous under ordinary conditions) coalesce 
to form nine grammes of liquid water is close to 34,100 gramme degrees, corre- 
sponding with about 1°5 volts; but the limiting value of the counter H.M.F., E-e’, 
in this case exceeds 4 volts. On the other hand, when water is decomposed by 
feeble currents so that the evolved gases are absorbed or occluded by the electrodes, 
or are attracted thereto forming air-films over their surfaces, the values of E —e’ may 
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