Experimental Electric Furnaces
By JEROME S. MARCUS, B. Sc. (Chem. Eng.)
An electric furnace is an apparatus for
the production of high temperatures
by electricity. The advantages of
such an apparatus are - the direct
application of heat to the material,
thus eliminating excessive losses by conduction thru the walls of a containing vessel;
the production of high temperature,
usually above those obtainable from fuel
in common uses; simple and accurate regulation,
giving absolute control of a process
and an economical use of power; and
finally, with sources of water-power, a low
There are several types of electric furnaces
in use. The general division are
the Induction type, and the Resistance type.
The purpose of this article is to give the
experimenter the simple construction details and operating principles of these furnaces.
Before going further, it is well to inform
the operator of any of these devices
to watch his fuses, as many will be blown
without the proper regulation of the rheostat
in Fig. 1. The experimenter will find
that a transformer is not necessary for a
small arc furnace, but in the case of the resistance
or induction types a higher voltage
than the ordinary lighting current is required
for good results.
The author has found the simplest rheostat
to be of the water-barrel type. A
wooden pail is first filled with strong salt
water. A metal plate in the bottom is attached
to one lead, which is well insulated;
a piece of rubber hose over the wire is excellent.
To this other lead is soldered a
metal electrode of any sort. The distance
between the plate and the electrode regulates
the current; the closer they are the
less the resistance. The experimenter may
fit up a support for his adjustable electrode
to suit his convenience. Fig. 2.
The arc furnace is by far the best for
the amateur. It is the simplest and cheapest
in construction, the easiest of operation
and regulation, the most economical and is
productive of higher temperature than the
A furnace which will give practical results
can be made from two blocks of slaked
lime, hollowed out and grooved for two
carbon electrodes, as shown. This apparatus
can be run on the ordinary lighting
current the same as an arc-light. To start
the arc in operation the carbons are touched
together and then drawn a small distance
apart, giving a steady arc. The material
to be melted is placed in the hollow beneath
the arc. To stop the arc, the carbons are
drawn far apart, thus breaking the arc.
The use of a rheostat or "ballast" improves
the steadiness of the arc. A transformer is
not necessary as was stated before.
Since an electric arc between carbon
wears down the positive electrode, adjustment
is frequently necessary in order to
maintain the flow of current. One carbon
should therefore be made loose in order to
feed it in as required.
The above apparatus is not one that the
experimenter cares to have as a permanent
part of his laboratory. Below is given a
description of a highly efficient furnace
with which any experiment can be readily
A clay flower pot is drilled to permit the
carbon to pass thru, and is lined with fire clay
or lime. The carbons are attached to
wooden blocks, as shown. The pot is set on
a board base with a circle of asbestos beneath.
The adjustment of the carbon is
made by means of threaded bolts moving
the blocks in grooves. A single pole, single
throw-knife switch may be mounted on the
base. A clay cover is placed over the pot
when in operation. The details are shown
in Figs. 4 A, B and C. One inch wood is
used for blocks and base.
A three-electrode arc furnace is shown
in Fig. 4-D. The positive electrode only
needs to be adjusted to keep the arc in the
center of the chamber.
The adjustment of the electrodes is eliminated if water cooled metal ones are
utilized in place of the carbon. These are
more expensive to make, however, and unless
carefully made soon come apart. A
copper disc is welded or brazed on the end
of a copper or brass tube. The cooling
water is introduced thru a small metal
pipe, see Fig. 5. An electrode can be made
of iron pipe with a cap screwed on the end
but is less efficient, owing to the high resistance
of the iron.
The resistance type of furnace depends
on the resistance offered by the material to
an electric current, for its source of heat.
This form of furnace is the one used in the
manufacture of carborundum, the smelting
of ores, especially aluminum and in the
refining of zinc.
The simplest resistance furnace consists
of a flower pot or other clay container, in
which the material is placed around two
carbon electrodes, as shown in Fig. 6. Very
often it is necessary to place a little granulated
carbon between the electrodes to start
the flow of current.
Another type of resistance furnace utilizes
the container as one electrode. A
graphite crucible is generally used, altho an
iron pot lined with fire clay in which a
large amount of carbon has been mixed,
may be used for the lower temperatures.
Only one connection is then made to the
carbon electrodes, of which there may be
one or several, depending on the size of
the furnace, the other connection being
made to the container itself. Higher
voltages than 110 are best for this type of
furnace, the amperage varying with the resistance
offered by the material. The carborundum
furnace at Niagara Falls runs on
potentials as high as 22,000 volts.
The induction furnace (Fig. 8) is not a
practical one for the experimenter. It requires
some of the molten material to start
it, and due to the high reactance resulting
from the distance between the primary and
secondary is uneconomical for a laboratory
device. The induction furnace operates as
a transformer, the secondary winding in
this case being the "charge," which is contained
in the circular channel A, and is
heated by the secondary current. The
amount of energy put into the secondary
can be varied by varying the applied primary
It is hoped that these few notes will
prove useful to the electrical experimenter.
There is a large number of unsolved problems
concerning the behavior of various
substances at high temperatures yet to be
worked out, and the results of some experimenter's
research may be, for all we can
tell, of great commercial or scientific value.
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