THE ELECTRIC FURNACE. 303 



the end walls of which project the electrodes C C, consisting of com- 

 posite bundles of carbon rods set in massive metal clamps M. The ' 

 space between the two electrodes is bridged by a conducting path of 

 coke A, which constitutes the distinct core of the furnace, and rele- 

 gates it to the class of which Borchers's furnace is a typical example. 

 This core is packed round with the raw material N, consisting of coke, 

 sand, sawdust, and common salt. The process of conversion is said 

 to be far from economical. 



A resistance furnace, based upon the fundamental principle of the 

 Nernst lamp, has been devised by Doctors Nernst and G laser. The 

 resistance, or heater, is cyiindrical, electrical connection with it at 

 the top and bottom being secured by an annular packing of some con- 

 ducting oxide held in place by iron clamps and l)olts. The hollow 

 cylinder is surrounded by a jacket of oxide loosely packed between 

 it and an outer cylindrical sheath, an arrangement which prevents 

 undue waste of heat. The heating cylinder, which consists of a mix- 

 ture of magnesia, calcium carbonate, alumina, and silica, is closed by 

 a lid, and the substance to be treated is either packed directly into it 

 or contained in a crucible located within it. In the former case the 

 cylinder is protected internally b}^ an additional lining of pure mag- 

 nesia, coated with graphite to give it an initial conductivity. 



The Cowles furnace, again, is typical of that class of resistance fur- 

 nace in which the path of high resistance consists of the material to be 

 treated and does not form part of the furnace proper. The Cowles 

 furnace first made its appearance in 1884, and takes several forms, all 

 more or less similar in general principle, but differing in such details 

 as affect the class of work for which they are intended. 



In its simplest form it consists of an oblong fire-brick structure, 

 provided with a lid, in w^liich are one or more vent holes to permit the 

 escape of the gases generated. Massive carbon electrodes are intro- 

 duced horizontally through the two ends of the furnace, electrical 

 connection with them being secured, in an earl}" form, ])y a species of 

 tubular gland through which each electrode passed and which was 

 filled with copper shot. In passing to and fro through these glands 

 the carbon rods set up a kind of rolling friction with the shot, and 

 fairly good electrical contact was thus established between them. 



A preliminary lining of granular charcoal was given to the furnace, 

 which, l)eing a bad conductor of heat, prevented undue loss due from 

 radiation and diffusion. Inside this lining, again, was packed the par- 

 tially conducting mass to be heated, forming a chain between the two 

 carbon electrodes. When the current was turned on this mass became 

 heated by the passage of the current through it, after the manner of 

 the carbon filament in the ordinary incandescent lamp. 



