will get the united effect of the whole, and the total current
will flow through the wire W, joining the extreme bars or positive
and negative poles of the battery. It must be borne in mind that
although the bismuth and antimony of this thermo-electric battery,
like the zinc and copper of the voltaic or chemico-electric
battery, are respectively positive and negative to each other, the
poles or wires attached to these metals are, on the contrary,
negative and positive. This peculiarity arises from the current
starting between the bismuth and antimony at the heated junction.

The internal resistance of a "thermo-electric pile" is, of course,
very slight, the metals being good conductors, and this fact gives
it a certain advantage over the voltaic battery. Moreover, it is
cleaner and less troublesome than the chemical battery, for it is
only necessary to keep at the required difference of temperature
between the hot and cold junctions in order to get a steady
current. No solutions or salts are required, and there appears to
be little or no waste of the metals. It is important, however, to
avoid sudden heating and cooling of the joints, as this tends to
destroy them.

Clammond, Gulcher, and others have constructed useful thermo-piles
for practical purposes. Figure 24 illustrates a Clammond thermo-
pile of 75 couples or elements. The metals forming these pairs are
an alloy of bismuth and antimony for one and iron for the other.
Prisms of the alloy are cast on strips of iron to form the
junctions. They are bent in rings, the junctions in a series
making a zig-zag round the circle. The rings are built one over the
other in a cylinder of couples, and the inner junctions are heated
by a Bunsen gas-burner in the hollow core of the battery. A gas-