THE ELECTRICITY OF HEAT.
In the year 1821 Professor Seebeck, of Berlin, discovered a third
source of electricity. Volta had found that two dissimilar metals
in contact will produce a current by chemical action, and Seebeck
showed that heat might take the place of chemical action. Thus, if
a bar of antimony A (fig. 22) and a bar of bismuth S are in
contact at one end, and the junction is heated by a spirit lamp to
a higher temperature than the rest of the bars, a difference in
their electric state or potential will be set up, and if the other
ends are joined by a wire W, a current will flow through the wire.
The direction of the current, indicated by the arrow, is from the
bismuth to the antimony across the joint, and from the antimony to
the bismuth through the external wire. This combination, which is
called a "thermo-electric couple," is clearly analogous to the
voltaic couple, with heat in place of chemical affinity. The
direction of the current within and without the couple shows that
the bismuth is positive to the antimony. This property of
generating a current of electricity by contact under the influence
of heat is not confined to bismuth and antimony, or even to the
metals, but is common to all dissimilar substances in their
degree. In the following list of bodies each is positive to those
beneath it, negative to those above it, and the further apart any
two are in the scale the greater the effect. Thus bismuth and
antimony give a much stronger current with the same heating than
copper and iron. Bismuth and selenium produce the best result, but
selenium is expensive and not easy to manipulate. Copper and
German silver will make a cheap experimental couple:--
POSITIVE
Bismuth
Cobalt
Potassium
Nickel
Sodium
Lead
Tin
Copper
Platinum
Silver
Zinc
Cadmium
Arsenic
Iron
Red phosphorus
Antimony
Tellurium
Selenium
NEGATIVE
Other things being equal, the hotter the joint in comparison with
the free ends of the bars the stronger the current of electricity.
Within certain limits the current is, in fact, proportional to
this difference of temperature. It always flows in the same
direction if the joint is not overheated, or, in other words,
raised above a certain temperature.
The electromotive force and current of a thermo-electric couple is
very much smaller than that given by an ordinary voltaic cell. We
can, however, multiply the effect by connecting a number of pairs
together, and so forming a pile or battery. Thus figure 23 shows
three couples joined "in series," the positive pole of one being
connected to the negative pole of the next. Now, if all the
junctions on the left are hot and those on the right are cool, we
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-
pipe seen in front leads to the burner, and the wires WW connected
to the extreme bars or poles are the electrodes of the pile.
Thermo-piles are interesting from a scientific point of view as a
direct means of transforming heat into electricity. A sensitive
pile is also a delicate detector of heat by virtue of the current
set up, which can be measured with a galvanometer or current
meter. Piles of antimony and bismuth are made which can indicate
the heat of a lighted match at a distance of several yards, and
even the radiation from certain of the stars.
Thermo-batteries have been used in France for working telegraphs,
and they are capable of supplying small installations of the
electric light or electric motors for domestic purposes.
The action of the thermo-pile, like that of a voltaic cell, can be
reversed. By sending a current through the couple from the
antimony to the bismuth we shall find the junction cooled. This
"Peltier effect," as it is termed, after its discoverer, has been
known to freeze water, but no practical application has been made
of it.
A very feeble thermo-electric effect can be produced by heating
the junction of two different pieces of the same substance, or
even by making one part of the same conductor hotter than another.
Thus a sensitive galvanometer will show a weak current if a copper
wire connected in circuit with it be warmed at one point.
Moreover, it has been found by Lord Kelvin that if an iron wire is
heated at any point, and an electric current be passed through it,
the hot point will shift along the wire in a direction contrary to
that of the current.