THEORY OF CONTACT ELECTROMOTIVE FORCE. 
65 
Marianini admits the spark, but I give it up altogether. Jacobi’s paper* convinces 
me I was in error as to that proof of the existence of a state of tension in the metals 
before contact (915. 956.). I need not therefore do more at present than withdraw 
my own observations. 
1807. I now proceed to address myself to the general argument, rather than 
to particular controversy, or to the discussion of cases feeble in power and doubtful 
in nature ; for I have been impressed from the first with the feeling that it is no weak 
influence or feeble phenomenon that we have to account for, but such as indicates a 
force of extreme power, requiring, therefore, that the cause assigned should bear 
some proportion, both in intensity and quantity, to the effects produced. 
1808. The investigations have all been made by aid of currents and the galvano- 
meter, for it seemed that such an instrument and such a course were best suited to an 
examination of the electricity of the voltaic pile. The electrometer is no doubt a 
most important instrument, but the philosophers who do use it are not of accord in 
respect to the safety and delicacy of its results. And even if the few indications 
as yet given by the electrometer be accepted as correct, they are far too general to 
settle the question of, whether contact or chemical action is the exciting force in 
the voltaic battery. To apply that instrument closely and render it of any force in 
supplying affirmative arguments to either theory, it would be necessary to construct 
a table of contacts, or the effects of contacts, of the different metals and fluids con- 
cerned in the construction of the voltaic pile, taken in pairs (1868.), expressing in such 
table both the direction and the amount of the contact force. 
1809. It is assumed by the supporters of the contact theory, that though the me- 
tals exert strong electromotive forces at their points of contact with each other, yet 
these are so balanced in a metallic circuit that no current is ever produced whatever 
their arrangement may be. So in Plate III. fig. 1 . if the contact force of copper and zinc 
is 10 and a third metal be introduced at m , the effect of its contacts, whatever 
that metal may be, with the zinc and copper at b and c, will be an amount of force 
in the opposite direction = 10. Thus, if it were potassium, its contact force at 
b might be 5 3 -, but then its contact force at c would be 15 : or if it were 
gold, its contact force at b might be 19, but then its contact force at c would be 
9 s-. This is a very large assumption, and that the theory may agree with the 
facts is necessary : still it is, I believe, only an assumption, for I am not aware of any 
data, independent of the theory in question, which prove its truth. 
1810. On the other hand, it is assumed that fluid conductors, and such bodies as 
contain water, or, in a word, those which I have called electrolytes (664. 823. 921.), 
either exert no contact force at their place of contact with the metals, or if they do 
exert such a power, then it is with this most important difference, that the forces are 
not subject to the same law of compensation or neutralization in the complete circuit, 
as holds with the metals (1809.). But this, I think I am justified in saying, is an as- 
* Philosophical Magazine, 1838, xiii. 401. 
MDCCCXL. 
K 
