bout 16 ft. long and 5 ft. wide by 5
ft. high, with massive brick end walls 2 ft. thick, through which are
built the carbon poles, consisting of bundles of 60 parallel 3-in.
carbon rods, each 3 ft. in length, with a copper rod let into the outer
end to connect it with a copper cap, which in turn is connected with one
of the terminals of the generating dynamo. The spaces between the
carbons of the electrode are packed tightly with graphite. In preparing
the furnace for use, transverse iron screens are placed temporarily
across each end, the space between these and the end walls being rammed
with fine coke, and that in the interior is filled to the level of the
centre of the carbon poles with the charge, consisting of 34 parts of
coke, with 54 of sand, 10 of sawdust and 2 of salt. A longitudinal
trench is then formed in the middle, and in this is arranged a
cylindrical pile of fragments of coke about 1/2 in. or more in diameter,
so that they form a core, about 21 in. in diameter, connecting the
carbon poles in the end walls. Temporary side walls are then built up,
the iron screens are removed, and a further quantity of charge is heaped
up about 3 ft. above the top of the furnace. An alternating current of
about 1700 amperes at 190 volts is now switched on; as the mass becomes
heated by the passage of the current the resistance diminishes, and the
current is regulated until after about 2 hours or less from starting it
is maintained constant at about 6000 amperes and 125 volts. Carbon
monoxide is given off and burns freely around the sides and top of the
furnace, tinged yellow after a time by the sodium in the salt mixed with
the charge. Meanwhile a shrinkage takes place, which is made good by the
addition of a further quantity of charge until the operation is
complete, usually in about 36 hours from the commencement. The current
is then switched off, and the side walls, after cooling for a day, are
taken down, the comparatively unaltered charge from the top is removed,
and the products are carefully extracted. These consist of the inner
carbon core, which at the temperature of the furnace will have been for
the most part converted into graphite, then a thin black crust of
graphite mixed with carborundum, next a layer of nearly pure
crystallized carborundum about a foot in thickness, then grey amorphous
carbide of silicon mixed with increasing proportions of unaltered
charge, and lastly, on the outside, the portion of the cha