Page:Text-book of Electrochemistry.djvu/314

Rh parallel rods of carbon are placed near the long side of the furnace, and each is connected at one of the short sides by means of a conducting cable with the source of electricity. In the neighbourhood of the opposite short side the two carbon electrodes are connected by a small rod or a small piece of compressed carbon powder, which is quickly used up, and the carbon electrodes are gradually drawn out of the furnace as the mass of material suffers progressive reaction.

Resistance Furnaces. The Carborundum Process.—Instead of conducting the current through the contents of the furnace, and thus heating the substance which is to undergo reaction, the electricity may be passed through a relatively large carbon resistance, which becomes hot, and passes the heat on to the material near it.

The simplest furnace of this type is that designed by Borchers (10). A thin carbon rod, C (Fig. 52), lies between two larger ones, A and B. The material to be heated surrounds C, and a strong current is passed from A to B. A and B should be so thick that the current density does not amount to more than 0.1 amp./sq. mm. The size of the rod C depends on the temperature to which it is desired to heat the mass. A red heat is obtained if the current density in C amounts to 0.5 amp./sq. mm. If it is ten times as large, a temperature can be attained at which calcium carbide can be produced, and if the current density reaches 10-15 amp./sq. mm., temperatures of 3000 to 3500° can be reached. Borchers has stated that there is no oxide which can resist reduction when the current density is 10 amp./sq. mm. The carbon electrodes A and B are introduced through the sides of the furnace, which is made of fire-proof material, and is covered in the usual way.