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.] So imagine a circular trough of fire-clay or other heat-resisting material filled with fragments of iron, or, it may be, with iron barely above melting-point, which has come from another furnace, where it underwent the previous process. Circling inside or outside this trough is an enormous coil of wire through which currents of electricity are alternating. That is the "primary" of a transformer, and the "secondary" is--the iron itself, in the trough. If it be, as it often is, in the form of scrap, or broken pieces, the heat will begin to show itself where the pieces touch each other. The currents generated in the trough, by the coil outside, will, of course, pass from piece to piece and the points of contact, since they offer the greatest resistance, will show signs of heat. This will increase until the pieces begin to melt. As the separate fragments merge into the molten mass the resistance will in one way decrease, for the imperfect contacts between the pieces will give place to the perfect contact throughout the mass of liquid metal. But for another reason--namely, the increase in heat--the resistance will increase. And all the while the alternations in the primary coil will be pumping currents, as it were, round and round the ring of molten iron. Whether the resistance increase or decrease, the current will do the opposite, so that heat will be generated whatever happens. For as resistance decreases current increases, and vice versa. And the slightest variation in the strength of the primary current will have its effect upon the secondary, and therefore on the heat generated. So, by simply regulating the primary current, the temperature of the metal can be controlled to a nicety. And such furnaces have the immense advantage that there is no possibility of deleterious substances in the fuel getting into and spoiling the metal, a thing which may very easily happen during the manufacture of high-class steels, alloys of iron in which the exact quantities, purity and proportions of the ingredients are of the utmost importance. Hence these "induction furnaces," as they are called, are frequently used quite apart from any question of utilising water-power. And they will probably be used still more as time goes on.