By Thomas W. Benson
was first noted by Seebeck, of Berlin,
that when a juncture of dissimilar
metals was heated a current of electricity
would flow. This phenomena
was studied by dimming, Sturgeon,
Pellaci and many others, for it gave promise
of providing a means for developing
electricity direct from heat, without the
use of steam engines and generators.
Altho no practical application has been
made of the discovery, beyond its application
to pyrometry for heat measurements,
several interesting devices can be easily
constructed to demonstrate the phenomena.
The most powerful currents are generated
by heating junctures of the more
crystalline metals, such as antimony and
bismuth, the other metals lying between
these in regard to the strength of the
current generated with a given rise in temperature.
The following list gives the
thermo-electric order of the more common
Any two of these metals when joined
together and the junction heated results
in a current flowing thru the exterior circuit
from the metal highest on the list to
that lower. The reverse is true should the
junction be cooled. The further apart the
metals are in the table the greater will be
the electromotive force.
It is seen that bismuth and antimony
will give a so-called "couple" having the
highest E.M.F. These are rather difficult
to obtain and handle, so for experimental
purposes we will substitute iron and German-
silver wire. These two metals are
employed in the construction of thermocouples
for electric pyrometers, and will
serve our purpose admirably.
The electro-dynamic effect of heat generated
currents can be shown by a device
similar to that illustrated in Fig. 1. Three
sides of the wire rectangle are composed
of iron, the fourth of German-silver. The
frame is suspended over a permanent magnet
and heat applied to one corner as shown.
The frame will turn thru an angle of 90°
due to the currents flowing in the loop.
A large number of turns of wire could
be made to form a heat compass. The
details of such a device are shown in Fig.
2. A small bundle of soft iron wire is
bound with paper glued into place. Take
a number of lengths of iron and German-silver
wire and wrap them in the same
direction on the core, in alternate coils as
shown. Twist the ends of the wire together
so that every other joint is on top.
The ends of the more distant coils being
connected together by a loop of wire, thus
forming a sling to support the compass.
When this apparatus is suspended over
a hot copper or brass plate, the currents
flowing around the windings will cause it
to act as a compass and the core will place
its axis in a line with the natural magnetic
field of the earth.
A form of thermo-electric motor can be
constructed as shown in Fig. 3, to demonstrate
electro-magnetic rotation. Here we
have a ring of iron wire attached to which
are a number of vertical German-silver
wires, bent over at the top and joined at
the center to a pivot. The latter rests in a
tiny depression in the pole of a horseshoe
magnet, allowing the cage to turn freely.
By applying a small flame to the frame at
the juncture of the G.S. and iron wires between
the legs of the magnet a current will
flow thru the vertical wires and rotation
will result from magnetic repulsion. The
device will attain quite a high speed if carefully
A thermo-battery can be cheaply made
that will deliver a fairly heavy current in
the following manner:" Take" a 1" pine
board 6" square and drill with small holes
at the intersection of lines drawn in both
directions 1/4" apart. The board is then
impregnated by placing in melted paraffin.
Alternate 2" lengths of iron and G.S. wire
are pushed into the holes and their ends
tightly twisted together on both sides of
the board, the end wires being led to binding
When the battery is stood with one side
in the sun, the other being shaded, an appreciable
E.M.F. will be generated. If laid
on a block of ice in the sun it will ring a
A somewhat similar form of thermo-pile,
comprising antimony and bismuth
bars, in the hands of Forbes and Milloni
led to the beautiful discovery of the polarization
Andrews noted that a current was generated
when platinum wires are inserted in
a bead of fused borax, potassium chlorid,
chlorids of potassium and strontium, iodid
of potassium, sulfate of soda, and even
when boracic acid is used. These results,
however, are due to a thermo-chemical
action and formed the basis for the work
of Edison and others on the heat cells.
The action differs from the true thermic
action of dissimilar metals.
Conversely we have electro-thermic effects
whereby heat and cold result from
an electric current passing between dissimilar
metals. The production of cold by
this means is rather startling and is known
as the Peltier effect.
Apparatus to demonstrate the latter effect
can be assembled as shown in Fig. 5. A
large thistle tube has connected to its
lower end a U-tube with a small bore, by
means of a length of rubber tubing. The
U-tube is partly filled with colored water.
A cork that fits the mouth of the thistle
tube has six holes bored thru it into which
are inserted short lengths of iron and
G.S. wire, about five strands of wire being
put in each hole. These are twisted together
to form thermo-couples and the cork inserted
in the mouth of the thistle tube. A little
paraffin wax melted over the cork will seal
the whole effectively.
When the positive and negative poles of
a battery (gravity type preferred) are connected
to the iron and G.S. wire respectively,
the junctures inside the tube will
drop in temperature and the air contracting
will cause the water in the open leg of the
U-tube to drop. For best results the current
should be regulated by a rheostat so
that it is not sufficient to heat the wire
used in the couples.
A current having a fairly high E.M.F.
and low amperage gives the best results.
The exact cause of the current generated
in a thermo-couple is still an unsolved
problem. It is not due to any oxidization
or chemical effect for it has been found
that a bar of the same metal with different
temperature of its opposite ends will give a
slight current, the cold portions becoming
negative, the hot parts positive. A thoro
understanding of the phenomena demands
more complete knowledge of matter structure
and the inter-atomic forces.
A fuller realization of the forces in action
may revolutionize present methods of
current generation and perhaps refrigeration.
Thus there is a very fine field open for
experimentation by the industrious student.
By experiment we learn many things here-tofore
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