76 DR. FARADAY’S EXPERIMENTAL RESEARCHES IN ELECTRICITY. (SERIES XVI.) 
current far stronger than that previously existing. If then, from the weaker current, 
the part of the effect due to chemical action be abstracted, how little room is there 
to suppose that any effect is due to the contact of the metals. 
1850. But a red nitric acid with platinum plates conducts a thermo current well, and 
will do so even when considerably diluted (1818.). When such red acid is used be- 
tween iron and platinum, the conducting power is such, that one half of the permanent 
current can be overcome by a counter thermo current of bismuth and antimony. Thus 
a sort of comparison is established between a thermo current on the one hand, and a 
current due to the joint effects of chemical action on iron and contact of iron and 
platinum on the other. Now considering the admitted weakness of a thermo current, 
it may be judged what the strength of that part of the second current due to contact 
can, at the utmost, be ; and how little it is able to account for the strong currents 
produced by ordinary voltaic combinations. 
1851. If for a clean iron wire one oxidized in the flame of a spirit-lamp be used, 
being associated with platinum in pure strong nitric acid, there is a feeble current, the 
oxide of iron being positive to the platinum, and the facts mainly as with iron. But the 
further advantage is obtained of comparing the contact of strong and weak acid with 
this oxidized wire. If one volume of the strong acid and four volumes of water be 
mixed, this solution may be used, and there is even less deflection than with the strong 
acid: the iron side is now not sensibly active, except the most delicate means be used 
to observe the current. Yet in both cases if a chemical action be introduced in place 
of the contact, the resulting current passes well, and even a thermo current can be 
made to show itself as more powerful than any due to contact. 
1852. In these cases it is safest to put the whole of the oxidized iron under the 
surface and connect it in the circle by touching it with a platinum wire ; for if the 
oxidized iron be continued through from the acid to the air, it is almost certain 
to suffer from the joint action of the acid and air at their surface of contact. 
1853. I proceeded to use a fluid differing from any of the former : this was solution 
of potassa, which has already been employed by De la Rive (1823.) with iron and 
platina, and which when strong has been found to be a substance conducting so well, 
that even a thermo current could pass it (1819.), and therefore fully sufficient to show 
a contact current, if any such exists. 
1854. Yet when a strong solution of this substance was arranged with silver and 
platinum, (bodies differing sufficiently from each other when connected by nitric or 
muriatic acid,) as in the former cases, a very feeble current was produced, and the 
galvanometer-needle stood nearly at zero. The contact of these metals therefore did 
not appear to produce a sensible current ; and, as I fully believe, because no electro- 
motive power exists in such contact. When that contact was exchanged for a very 
feeble chemical action, namely, that produced by interposing a little piece of paper 
