ELECTRIC HEATING
The light of the electric incandescent lamp is produced by the heating
to incandescence of a thin filament of metal or carbon, and the heat
itself is produced by the electric current forcing its way through
the great resistance opposed to it by the filament. In such lamps the
amount of heat produced is too small to be of much practical use, but
by applying the same principle on a larger scale we get an effective
electric heater.
The most familiar and the most attractive of all electric heaters is
the luminous radiator. This consists of two or more large incandescent
lamps, having filaments of carbon. The lamps are made in the form of
long cylinders, the glass being frosted, and they are set, generally
in a vertical position, in an ornamental case or frame of metal. This
case is open at the front, and has a metal reflector behind. The
carbon filaments are raised to an orange-red heat by the passage of
the current, and they then radiate heat rays which warm the bulbs and
any other objects in their path. The air in contact with these heated
bodies is warmed, and gradually fills the room. This form of heater,
with its bright glowing lamps, gives a room a very cheerful appearance.
In the non-luminous heaters, or “convectors” as they are called, the
heating elements consist of strips of metal or wires having a very
high resistance. These are placed in a frame and made red-hot by the
current. Cold air enters at the bottom of the frame, becomes warm by
passing over the heating elements, and rises out at top and into the
room. More cold air enters the frame and is heated in the same way, and
in a very short time the whole of the air of the room becomes warmed.
The full power of the heater is used in the preliminary warming of the
room, but afterwards the temperature may be kept up with a much smaller
consumption of current, and special regulating switches are provided
to give different degrees of heat. Although these heaters are more
powerful than the luminous radiators, they are not cheerful looking;
but in some forms the appearance is improved by an incandescent lamp
with a ruby glass bulb, which shines through the perforated front of
the frame.
The Bastian, or red glow heater, has thin wires wound in a spiral and
enclosed in tubes made of quartz. These tubes are transparent both
to light and heat, and so the pleasant glow of the red-hot wire is
visible. A different type of heater, the hot oil radiator, is very
suitable for large rooms. This has a wire of high resistance immersed
in oil, which becomes hot and maintains a steady temperature.
Electric cooking appliances, like the heaters just described, depend
upon the heating of resistance wires or strips of metal. The familiar
electric kettle has a double bottom, and in the cavity thus formed is
placed the resistance material, protected by strips of mica, a mineral
substance very largely used in electrical appliances of all kinds
on account of its splendid insulating qualities. Electric irons are
constructed in much the same way as kettles, and sometimes they are
used with stands which cut off the current automatically when the iron
is laid down upon them, so that waste and overheating are prevented.
There are also a great many varieties of electric ovens, grillers,
hot-plates, water-heaters, glue-pots, and foot and bed warmers. These
of course differ greatly in construction, but as they all work on the
same principle there is no need to describe them.
Electric hot-plates are used in an interesting way in Glasgow, to
enable the police on night duty to have a hot supper. The plates are
fitted to street telephone signal boxes situated at points where a
number of beats join. By switching on current from the public mains the
policemen are able to warm their food and tea, and a supper interval of
twenty minutes is allowed. Even policemen are sometimes absent-minded,
and to avoid the waste of current and overheating of the plate that
would result if a “bobby” forgot to switch off, an arrangement is
provided which automatically switches off the current when the plate is
not in use.
[Illustration: FIG. 23.--Diagram to illustrate principle of Electric
Furnace.]
We must turn now to electric heating on a much larger scale, in the
electric furnaces used for industrial purposes. The dazzling brilliance
of the light from the electric arc lamp is due to the intense heat
of the stream of vaporized carbon particles between the carbon rods,
the temperature of this stream being roughly about 5400° F. This
great heat is made use of in various industries in the electric arc
furnace. Fig. 23 is a diagram of a simple furnace of this kind. A
is a vertical carbon rod which can be raised or lowered, and B is
a bed of carbon, forming the bottom of the furnace, and acting as
a second rod. A is lowered until it touches B, the current, either
continuous or alternating, is switched on, and A is then raised. The
arc is thus struck between A and B, and the material contained in the
furnace is subjected to intense heat. When the proper stage is reached
the contents of the furnace are drawn off at C, and fresh material
is fed in from above, so that if desired the process may be kept
going continuously. Besides the electric arc furnace there are also
resistance furnaces, in which the heat is produced by the resistance of
a conductor to a current passing through it. This conductor may be the
actual substance to be heated, or some other resisting material placed
close to it.
It will be of interest to mention now one or two of the uses of
electric furnaces. The well-known substance calcium carbide, so much
used for producing acetylene gas for lighting purposes, is a compound
of calcium and carbon; it is made by raising a mixture of lime and coke
to an intense heat in an electric furnace. The manufacture of calcium
carbide is carried on on a very large scale at Niagara, with electric
power obtained from the Falls, and at Odda in Norway, where the power
is supplied by the river Tysse. Carborundum, a substance almost as hard
as the diamond, is largely used for grinding and polishing purposes.
It is manufactured by sending a strong current through a furnace
containing a core of coke surrounded by a mixture of sand, sawdust, and
carbon. The core becomes incandescent, and the heating is continued
until the carbon combines with the sand, the process taking about
a day. Graphite, a kind of carbon, occurs naturally in the form of
plumbago, which is used for making black lead pencils. It is obtained
by mining, but many of the mines are already worked out, and others
will be exhausted before long. By means of the electric furnace,
graphite can now be made artificially, by heating anthracite coal,
and at Niagara a quantity running into thousands of tons is produced
every year. Electric furnaces are now largely employed, particularly
in France, in the production of the various alloys of iron which are
used in making special kinds of steel; and they are used also to a
considerable extent in the manufacture of quartz glass.
For many years past a great deal of time and money has been spent in
the attempt to make artificial diamonds. Quite apart from its use in
articles of jewellery, the diamond has many very important industrial
applications, its value lying in its extreme hardness, which is not
equalled by any other substance. The very high price of diamonds
however is at present a serious obstacle to their general use. If they
could be made artificially on a commercial scale they would become
much cheaper, and this would be of the greatest importance to many
industries, in which various more or less unsatisfactory substitutes
are now used on account of their much smaller cost. Recent experiments
seem to show that electricity will solve the problem of diamond making.
Small diamonds, one-tenth of an inch long, have been made in Paris
by means of the electric arc furnace. The furnace contains calcium
carbide, surrounded by a mixture of carbon and lime, and the arc,
maintained by a very powerful current, is kept in operation for several
hours. A black substance, something like coke, is formed round the
negative carbon, and in this are found tiny diamonds. The diamonds
continue to increase slowly in size during the time that the arc is
at work, and it is estimated that they grow at the rate of about
one-hundredth of an inch per hour. So far only small diamonds have
been made, but there seems to be no reason why large ones should not be
produced, by continuing the process for three or four days.
A chapter on electric heating would not be complete without some
mention of electric welding. Welding is the process of uniting two
pieces of metal by means of a combination of heat and pressure, so
that a strong and permanent joint is produced. The chief difficulty
in welding is that of securing and keeping up the proper temperature,
and some metals are much more troublesome than others in this respect.
Platinum, iron, and steel are fairly easy to weld, but most of the
other metals, and alloys of different metals, require very exact
regulation of temperature. It is almost impossible to obtain this
exact regulation by ordinary methods of heating, but the electric
current makes it a comparatively easy matter. The principle of ordinary
electric welding is very simple. The ends of the two pieces of metal
are placed together, and a powerful current is passed through them.
This current meets with a high resistance at the point of contact
of the two pieces, and so heat is produced. When the proper welding
temperature is reached, and the metal is in a sort of pasty condition,
the two pieces are pressed strongly together, and the current is
switched off. The pieces are now firmly united together. The process
may be carried out by hand, the welding smith switching the current
on and off, and applying pressure at the right moment by means of
hydraulic power. There are also automatic welders, which perform the
same operations without requiring any manual control. Alternating
current is used, of low voltage but very high amperage.
Steel castings are sometimes found to have small defects, such as
cracks or blow-holes. These are not discarded as useless, but are made
quite sound by welding additional metal into the defective places by
means of the electric arc. The arc is formed between the casting and a
carbon rod, and the tremendous heat reduces the surface of the metal to
a molten condition. Small pieces or rods of metal are then welded in
where required.