CHAPTER XI.

ELECTRIC HEAT.

WE have already stated that the electric current is capable of producing heat as well as light. Indeed, the electric light is due to the high temperature which is excited either in the vapours of the luminous arc or the incandescent filament of a lamp by the passage of the current through it: and the reason why an electric lamp does not give off much heat is that the heated mass is very small. The temperature of the arc is high enough to fuse platinum or steel, but the body of matter carrying the heat is only a puff of carbon dust. Again, the filament of an incandescent light is raised to a temperature of over 1000° Fah., but it weighs only a few grains. It is possible, however, by both these means to generate a quantity of heat sufficient for useful purposes.

By employing powerful currents and massy carbonrods, Dr. C. W. Siemens has applied the electric arc to the fusion of metals. He has in fact constructed an electric furnace, which is illustrated in Fig. 78. It consists of a fire-clay furnace, having a plumbago crucible inside. A short carbon rod enters the bottom of this crucible by a hole, and serves as the positive electrode of the current, that is to say, the carbon

connected to the positive pole of the electric generator. Through the lid, which is slightly raised in the sketch, a corresponding carbon rod, serving as the negative electrode, enters the crucible. This rod, or rather bundle of rods, is suspended from one end of a balance beam, from the other end of which hangs a soft iron rod, which enters the hollow of a coil or bobbin of insulated copper wire. The lower part of this iron core dips into a well of glycerine or other viscous liquid, so that it will have a smooth motion up and down when the balance beam moves. A movable weight serves to balance the beam so as to give a proper length of arc between the two carbon rods in the crucible. The current is conveyed to the two carbons by wires, and the arc established by it is so powerful that comparatively large masses of copper, platinum, nickel, and tungsten, as well as the most refractory earths, are in a short time fused by it. The furnace is rendered self-regulating by connecting the solenoid or bobbin in a derived circuit or by-path from the arc, so that a portion of the current is diverted through it. Then if the arc becomes too long, and its resistance too great, more of the current is diverted through the solenoid, and this sucks up the soft iron core into the bobbin, thereby raising that end of the balance beam and lowering the other which carries the upper carbon. The arc is thus shortened again to its normal length. Contrarily, if the arc becomes too short, and its resistance consequently diminished, less current flows in the solenoid and the core sinks, pulling down the beam at its own end and raising the carbon at the other.

In this electric furnace Dr. Siemens can melt several pounds of old files in fifteen minutes with the current

from one of his dynamo-electric machines. To melt steel a temperature of 1,800° C. is required. As many as 20 lbs. of steel have been melted in one charge, the time being an hour. Copper is nearly all vaporised when subjected to the heat of the furnace. The advantages of the latter are that a very high temperature can be obtained, the highest on earth as far as we know, and the heat is developed in the material itself instead of having to traverse the walls of the containing vessel. It will therefore probably be useful for certain special purposes. Platinum wires rendered incandescent by passing the electric current through them have, as we have stated, for some years been employed in cauterising sores and in removing tumours by the bloodless process of searing.

In 1873 Dr. Onimus cauterised the lachrymal gland in several persons, one after another, from a single charge in a Planté secondary cell; and quite recently Professor Buchanan, of Glasgow, removed a tumour from a boy by help of the charge in a small Faure accumulator, and without shedding blood.

Mr. Lane-Fox has devised an electric egg or water boiler which is simply a hollow canteen of metal with double sides; the space between containing a coil of German silver wire, which is properly insulated from the walls and connected to terminals outside the boiler. The wires supplying the current are also brought to these terminals, and the current heats the wire and boils the water within the well of the canteen. The current diverted from an ordinary incandescent lamp will heat a pint of water in ten or fifteen minutes.

Heated platinum wires are also used to fire fuses in

blasting operations, and in exploding submarine mines and torpedoes. The wire is enclosed in a small priming of gunpowder, and when the current is sent through it by the instantaneous closing of the battery circuit the priming is fired and explodes the charge. Instead of wire a semi-conducting powder such as carbon dust or sulphide of copper is heated to redness by the current and ignites the priming. This powder is usually placed so as to fill up a little gap or breach in the copper wires forming the circuit of the battery. The fuse of Professor Abel is one of the best, and is a mixture of sub-phosphide of copper, sub-sulphide of copper, and chlorate of potash. The ingredients are pounded together in a mortar and made into a paste with alcohol, which is afterwards dried away. Abel's fuse is largely used in this country for mining purposes. It can be fired either by the current from a battery, or the spark from a magneto-electric exploder, which consists of a "horse-shoe" magnet having coils of wire bound round its poles, and an armature of soft iron mounted on a pivot in front. On suddenly striking down a handle the armature is jerked away from the poles of the magnet and a sudden charge of electricity excited in the coils. This appears in the form of a spark which is conducted to the fuse by insulated wires.

Gas jets are lighted by means of small platinum spirals of wire heated by the passage of the current, and also by an induction spark leaping across the air space between two metal points. The sparking gaslighter consists of a long hollow rod containing within it a small induction coil with auxiliary battery, and a contact maker operated by a press button. Wires