intense heat and resist the cutting
action of hot metal and slag. For a roof, a low dome of fire brick
is provided. The shell and lining is pierced in front for a pouring
spout, and on either side by doors, through which the raw material
is charged.

Two or three carbon "electrodes"--18-in. cylinders of specially
prepared coke or graphite--extend through holes in the roof. Electrical
connections are made to the upper ends, and a very high current
sent through them. This causes tremendous arcs to form between
the lower ends of the electrodes and the metal below, and these
electric arcs are the only source of heat in this style of furnace.

Electric furnaces can be used to do the same work as is done in
crucible furnaces--that is to say, merely melt a charge of carefully
selected pure raw materials. On the other hand it can be used to
produce very high-grade steel from cheap and impure metal, when
it acts more like an open-hearth furnace. It can push the refining
even further than the latter furnace does, for two reasons: first
the bath is not swept continuously by a flaming mass of gases;
second, the temperature can be run up higher, enabling the operator
to make up slags which are difficult to melt but very useful to
remove small traces of impurities from the metal.

Electric furnaces are widely used, not only in the iron industry,
but in brass, copper and aluminum works. It is a useful melter of
cold metal for making castings. It can be used to convert iron
into steel or vice versa. Its most useful sphere, however, is as a
refiner of metal, wherein it takes either cold steel or molten steel
from open hearth or bessemer furnaces, and gives it the finishing
touches.

[Illustration: FIG. 5.--"Slagging off" an electric furnace.]

[Illustration: FIG. 6.--Pouring the ingots.]

As an illustration of the furnace reactions that take place the
following schedule is given, showing the various stages in the
making of a heat of electric steel. The steel to be made was a
high-carbon chrome steel used for balls for ball bearings:

6-TON HEROULT FURNACE

11:50 A.M.--Material charged:
Boiler plate 5,980 lb.
Stampings 5,991 lb.
-----------
11,971 lb.
Limestone 700 lb.
12:29 P.M.--Completed charging (current switched on).
3:20 P.M.--Charge melted down.
Preliminary analysis under black slag.
Analysis:
Carbon Silicon Sulphur Phosphorus Manganese