78 DR. FARADAY’S EXPERIMENTAL RESEARCHES IN ELECTRICITY. (SERIES XVI.) 
contact theory, would utterly annihilate the force of the previous experimental re- 
sults, though it would not enable that theory to give a reason for the activity of, and 
the existence of a current in, the pile : but which, if in error, would leave the contact 
theory utterly defenceless and without foundation. 
1860. A supporter of the contact theory may say that the various conducting elec- 
trolytes used in the previous experiments are like the metals ; i. e. that they have an 
electromotive force at their points of contact with the metals and other solid conduc- 
tors employed to complete the circuit ; but that this is of such consistent strength at 
each place of contact, that, in a complete circle, the sum of the forces is 0 (1809.). 
The actions at the contacts are tense electromotive actions, but balanced, and so 
no current is produced. But what experiment is there to support this statement? 
where are the measured electromotive results proving it (1808.) ? I believe there are 
none. 
1861. The contact theory after assuming that mere contacts of dissimilar sub- 
stances have electromotive powers, further assumes a difference between metals and 
liquid conductors (1810.) without which it is impossible that the theory can explain 
the current in the voltaic pile : for whilst the contact effects in a metallic circuit are 
assumed to be always perfectly balanced, it is also assumed that the contact effects of 
the electrolytes or interposed fluid with the metals are not balanced, but are so far 
removed from anything like an equilibrium, as to produce most powerful currents, 
even the strongest that a voltaic pile can produce. If so, then why should the solu- 
tion of sulphuret of potassium be an exception? it is quite unlike the metals : it does 
not appear to conduct without decomposition ; it is an excellent electrolyte, and an 
excellent exciting electrolyte in proper cases (1880.), producing most powerful cur- 
rents when it acts chemically ; it is in all these points quite unlike the metals, and, in 
its action, like any of the acid or saline exciting electrolytes commonly used. How 
then can it be allowed that, without a single direct experiment, and solely for the 
purpose of avoiding the force of those which are placed in opposition, we should sup- 
pose it to leave its own station amongst the electrolytes, and class with the metals ; 
and that too, in a point of character which, even with them, is as yet a mere assump- 
tion (1809.). 
1862. But it is not with the sulphuret of potassium alone that this freedom must 
be allowed ; it must be extended to the nitrous acid (1843. 1847-), to the nitric acid 
(1849, &c.), and even to the solution of potash (1854.) ; all these being of the class 
of electrolytes, and yet exhibiting no current in circuits where they do not occasion 
chemical action. Further, this exception must be made for weak solutions of sul- 
phuret of potassium (1842.) and of potassa (1856.), for they exhibit the same phe- 
nomena as the stronger solutions. And if the contact theorists claim it for these 
weak solutions, then how will they meet the case of weak nitric acid which is not 
similar in its action on iron to strong nitric acid (1977-)? but can produce a powerful 
current ? 
