ON THERMO-ELECTRIC CURRENTS IN CIRCUITS OF ONE METAL. 175 
tween the metals, but in both cases from hot to cold across the joint, and 
were in each case about thirty times as great as the currents given by the 
thermo-electric difference between the metals. 
It was found on examining the Table, that wherever copper appeared in con- 
junction with any other of the metals named, the direction of the loose-con- 
tact current could invariably be determined by the following rule :—When 
the copper was the hot wire, the current flowed from the copper to the other 
metal across the joint; but when copper was the cold metal, the current flowed 
from the other metal to the copper, or in both eases from hot to cold. 
Exactly the contrary was found wherever iron appeared in conjunction 
with any of the five metals but copper; the current then always flowed from 
cold to hot. Two copper wires alone gave the largest deflection, of about 220 
diyisions ; and two iron wires alone gave the next largest of those obtained 
where single metals only were used, but of course in the opposite direction 
to the deflection from copper. 
It was then perceived that all these results would be explained if, the thin 
coating of oxide on the copper wire might be regarded as a conductor with a 
hot and cold junction, and endowed with thermo-electric properties far more 
positive than the iron, while at the same time the coating of oxide on the 
iron wire would have to be regarded as far more negative than the copper. 
It was, however, difficult to suppose that two bodies so similar in some re- 
spects as the oxides of copper and iron should be at opposite extremities of 
the thermo-electric scale, but the following direct experiment left no doubt 
on my mind. 
A little spiral was made of platinum wire, and a small quantity of oxide of 
copper laid upon it, and held in a flame till white-hot ; another platinum wire 
was then dipped in the melted mass, when a strong current was at once ob- 
served from the hot to the cold wire, as if a loose contact had been made 
between two copper wires. When either of the oxides of iron was tested in 
a similar manner, a strong current was obtained from the cold to the hot 
platinum wire, as if a loose contact had been made between two iron wires. 
I do not yet know positively what the substances are which, interposed 
between silver and platinum and gold wires, give rise to the loose-contact 
currents, but I feel no doubt that these are as much thermo-electric currents 
as those given by the oxides of copper and iron, and are produced in a circuit 
composed of the metal and a very thin hot film, of which the two surfaces are 
unequally heated. 
There are, however, some good reasons for doubting whether electrolytes 
can be included in a true thermo-electric series, and I consulted many autho- 
rities with reference to this point. Seebeck himself includes many electrolytes 
in his thermo-electric scale, and places acids below bismuth, a result con- 
firmed lately by Gore (in 1857); he also places certain salts above antimony, 
a result subsequently confirmed by Andrews of Belfast in 1837. This 
gentleman observed that the tension produced by the salts between the wires 
was about equal to that between a platinum and silver plate in dilute sulphuric 
acid, and that the metals used as electrodes did not influence the deflection. 
He considered the current certainly due to a thermo-electric action. 
Faraday in 1833 discovered what Becquerel subsequently called pyro-elec- 
tric currents ; the currents were in different directions with different substances 
used, and some, if not all, were of the same nature as those I have described. 
Leroux and Buff obtained currents where glass acted as the electrolyte. 
Leroux considered them thermo-electric, and Buff chemical effects. Buff 
also attributes some of the electrical phenomena connected with flame to a 
