The furnaces are in shape much like those in which iron is smelted with coal—namely, tall chimney-like structures at the bottom of which is the fire. In the "arc furnaces" there are, passing in through the side, near the bottom, a number of electrodes, and between these a series of arcs are formed. Coke and ironstone are thrown in from the top into this region of intense heat, and there the iron is liberated from the oxygen with which it is combined in the ore. Liberated, it flows out through a spout at one side of the furnace.
But the question will arise in the reader's mind: Why is coke needed in an electric furnace? It is for metallurgical reasons. The heat of the arc loosens the bonds between the iron and oxygen, but it needs the presence of some carbon to tempt the oxygen atoms away. Therefore coke, as the most convenient form of carbon, has to be there. It is there, however, in much smaller quantity than it would be in an ordinary furnace. It is not there as fuel, but simply as the "counter-attraction" to draw the oxygen atoms away from their old love.
The arc is also used for welding pieces of iron together, for which purpose it is eminently suitable, since what is wanted is intense heat at a particular point. But perhaps the reader will be wondering by this time what the heat of the arc is. It has been repeatedly referred to as "intense," but something more definite may be demanded. In theory it is unlimited. Apply more pressure—more volts, that is—thereby driving more current across, and the temperature will rise. It is only a question of making dynamos large enough, and driving them fast enough, and any temperature is possible. But there are practical difficulties which limit the degree of heat. One is the melting-point of the furnace itself. Fire-clay melts at about 1700° to 1800° C. So in a furnace which has to be lined with fire-clay that is about the limit.
In welding two pieces of iron together, the iron, of course, defines what the limit shall be. It needs to be heated to "welding heat" and no more—that is, a little short of melting—so that the parts to be joined are soft, and, with a little hammering, will join thoroughly together. If too much heat were to be applied the parts would melt away. But the heat of the arc can be controlled by simply varying the current, and so the right heat can be applied at the right place, than which little more is wanted.
One very simple way of doing this is for the workman to hold one of the "electrodes"—a rod of carbon suitably insulated—in his hand. The current is led to it through a flexible wire. The iron itself is made the other electrode by being gripped in a vice which is itself insulated but connected to the source of current. Thus on bringing the point of his rod near to the part to be heated the man causes an arc to be created there. By moving the rod he can move the arc about, heating one part more than another, distributing his heat if he wants to do so over a larger area, or keeping it to a small one, just as he wills. On reaching the right heat the rod is withdrawn, the arc destroyed, and the iron can be hammered just as if it had been heated in a fire.
Yet another way still is known as "resistance" welding. In it an enormous current at an extremely low voltage is used. The fundamental principle is the same, since the heat is formed by forcing current past a point over which it is reluctant to pass. That point of poor conductivity is the ends of the two bars to be joined. They are placed just touching, but since an imperfect contact like that always offers considerable resistance to the flow of a current, the passing current needs only to be made large enough for great heat to be generated.
This is exceedingly pretty to watch. We will suppose that the article to be operated upon is the tyre of a wheel. The bar of iron has already been bent by rollers into the correct curve and the two ends are touching. Brought to the machine, it is gripped, each side of the junction, in the jaws of an insulated vice and the current is turned on. In a few seconds the place where the two ends are just touching begins to glow. Rapidly it increases in brightness until in about half-a-minute it is at welding heat. Then one vice, which is movable, is forced along a little by a screw, so that the ends are pressed firmly together, a little judicious hammering meanwhile helping to complete the job. Then the current is switched off and the complete tyre taken out of the machine. The current used has a force comparable with that which operates domestic electric bells, but in volume it is thousands of amperes. Alternating current is used, and it is obtained from a transformer or induction coil. In such a case the primary part of the coil is made of many turns of fine wire, so that little current passes through it, while the secondary part is but one or two turns of thick bar. Thus the voltage generated in the secondary is very little, but since the secondary has an almost negligible resistance the current caused by that small voltage is enormous. Such an arrangement is in industrial realms generally called a transformer, the term induction coil being employed more for those things of a similar nature intended for the laboratory. The one just described is, moreover, a "step-down" transformer, since it lowers the voltage, to distinguish it from "step-up" transformers, which raise the voltage.
And the "resistance" principle is also applied in another way to large furnaces, such as those for refining iron. In these the resistance of the iron itself is utilised to generate the heat. Of course, it should be well understood, heat is always generated in everything through which current flows. There is no perfect conductor, and so every conductor is more or less heated by the passage of current through it. Some energy needs to be expended to drive current, even along large copper wires, and that energy must be turned into heat in the wires. If the same volume of current be forced along iron wires of the same size, the heat will be greater, since iron is but a poor conductor compared with copper, the relation being about as one to six. And if the iron be hot the resistance will be still more, for it stands to reason that when heated the molecules, being farther apart, will be the less easily able to exchange corpuscles. We have the best reasons for believing, as has been suggested already, that a current of electricity is but a flow of corpuscles, and so we are not surprised to hear that, as a general rule, the hotter a thing is the less does it conduct electricity.
By permission of Cambridge Scientific Inst. Co., Ltd., Cambridge, Eng.
Measuring Heat at a Distance
This wonderful instrument, the Fery Radiation Pyrometer, although itself some distance away from the furnace, is telling the temperature of its hottest part.