496 M. F. Braun on the 
experiments of Magnus, pieces of the same metal but of 
different temperature produce a thermoelectric current when 
brought quickly into contact. Only with lead could this re- 
sult not be observed with certainty. But in lead, as Le Roux 
showed, the Thomson-effect is nearly or quite zero. In 
general, those metals which show a large Thomson-effect 
show also the further peculiarity that, upon heating a single 
wire the ends of which are kept at a constant temperature, a 
thermoelectric current is produced, the intensity (and even 
direction) of which often varies with insignificant changes in 
the distribution of the temperature or the structure of the 
wire. 
These currents, I may at once mention, are so energetic, 
and especially so variable, particularly if the temperature is 
raised to a red heat, that I have had to take precautions with 
my thermopiles that there should not be any great differences 
of temperature in the solid metal. With lead only have I 
been unable to observe with certainty any such currents, 
even when the solid metal was plunged into molten lead. 
According to an experiment of Magnus’s, often quoted, 
which for short I will call the Magnus experiment, currents 
are altogether absent in mercury. 
It is an obvious explanation to refer the currents, produced 
on bringing together a cold and hot metal, to differences in 
structure (although we must then ascribe some influence 
either to time or to distribution of temperature), and to class 
them together with the Thomson effect—that is, the deviations 
which the thermoelectric force shows from proportionality 
with the temperature-difference. If certain metals give no 
thermo-current either in the Magnus experiment or when a 
wire, whose ends are maintained at a constant temperature, is 
heated, this may be explained in two ways: either changes 
of potential take place on both sides of the point heated, 
which, however, must decrease equally in both directions, or 
no thermoelectric excitation takes place, because, in conse- 
quence of the nature of the metal, differences in structure are 
impossible. 
To this latter, although more special, assumption it seems 
to me we shall be most inclined. It is to be assumed that 
other molten metals will behave as mercury does, which, in 
fact, I have found to be the case (cf. § 7). 
2. If we regard the Magnus experiment in this second 
way, it would follow that the thermoelectric force between 
molten metals must be proportional to the difference in 
absolute temperature between their points of contact. But 
this would show the possibility of measuring absolute tem- 
