72 DR. FARADAY’S EXPERIMENTAL RESEARCHES IN ELECTRICITY. (SERIES XVI.) 
those already obtained ; and if the reasoning then urged was good, it will now follow 
that the contact of platinum and nickel with each other, or of either with any of the 
different metals or solid conductors introduced at x, is entirely without electromo- 
tive force*. 
1837- Many other pairs of metals were compared together in the same manner; 
the solution of sulphuret of potassium connecting them together at one place, and 
their mutual contact doing that office at another. The following are cases of this 
kind : iron and gold ; iron and palladium ; nickel and gold ; nickel and palladium ; 
platina and gold ; platina and palladium. In all these cases the results were the 
same as those already given with the combinations of platinum and iron. 
1838. It is necessary that due precaution be taken to have the arrangements in an 
unexceptionable state. It often happened that the first immersion of the plates gave 
deflections ; it is, in fact, almost impossible to put two plates of the same metal into 
the solution without causing a deflection ; but this generally goes off very quickly, 
and then the arrangement maybe used for the investigation (1826.). Sometimes 
there is a feeble but rather permanent deflection of the needle; thus when platinum 
and palladium were the metals, the first effect fell and left a current able to deflect 
the galvanometer-needle 3°, indicating the platinum to be positive to the palladium. 
This effect of 3°, however, is almost nothing compared to what a mere thermo 
current can cause, the latter producing a deflection of 60° or more ; besides which, even 
supposing it an essential effect of the arrangement, it is in the wrong direction for 
the contact theory. I rather incline to refer it to that power which platinum and other 
substances have of effecting combination and decomposition without themselves en- 
tering into union ; and I have occasionally found that when a platinum plate has been 
left for some hours in a strong solution of sulphuret of potassium (1812.) a small 
quantity of sulphur has been deposited upon it. Whatever the cause of the final 
feeble current may be, the effect is too small to be of any service in support of the 
contact theory ; while, on the other hand, it affords delicate and, therefore, strong 
indications in favour of the chemical theory. 
1839. A change was made in the form and arrangement of the cup D, fig. 2, so as 
to allow of experiments with other bodies than the metals. The solution of sulphuret 
of potassium was placed in a shallow vessel, the platinum plate was bent so that the 
immersed extremity corresponded to the bottom of the vessel; on this a piece of 
loosely folded cloth was laid in the solution, and on that again the mineral or other 
substance to be compared with the platinum ; the fluid being of such depth that only 
part of that substance was in it, the rest being clean and dry ; on this portion the 
platinum wire, which completed the circuit, rested. The arrangement of this part of 
* One specimen of nickel was, on its immersion, positive to platinum for seven or eight minutes, and then 
became neutral. On taking it out it seemed to have a yellowish tint on it, as if invested by a coat of sulphuret ; 
and I suspected this piece had acted like lead (1885.) and bismuth (1895.). It is difficult to get pure and also 
perfectly compact nickel ; and if porous, then the matter retained in the pores produces currents. 
