In addition to the above gaseous rectifiers of oscillations it has been found that several crystals, such as carborundum (carbide of silicon), hessite, anastase and many others possess a unilateral conductivity and enable us to rectify trains of oscillations into continuous currents which can affect a telephone.
Wheels of carborundum are also used.
A somewhat impure silicon (containing 90-98% of the element) is made by the Carborundum Company of Niagara Falls (United States Patents 745 122 and 842273, 1908) by heating coke and sand in an electric furnace.
Many of the furnaces now in constant use depend mainly on this principle, a core of granular carbon fragments stamped together in the direct line between the electrodes, as in Acheson's carborundum furnace, being substituted for the carbon pencils.
Again, the construction of electric furnaces may often be exceedingly crude and simple; in the carborundum furnace, for example, the outer walls are of loosely piled bricks, and in one type of furnace the charge is simply heaped on the ground around the carbon resistance used for heating, without containing-walls of any kind.
Calcium carbide, graphite, phosphorus and carborundum are now extensively manufactured by the operations outlined above.
For the theory and elemental laws of electro-deposition see Electrolysis; and for the construction and use of electric generators see Dynamo and Battery: Electric. The importance of the subject may be gauged by the fact that all the aluminium, magnesium, sodium, potassium, calcium carbide, carborundum and artificial graphite, now placed on the market, is made by electrical processes, and that the use of such processes for the refining of copper and silver, and in the manufacture of phosphorus, potassium chlorate and bleach, already pressing very heavily on the older non-electrical systems, is every year extending.
As an artificial product, graphite is well known as dark lustrous scales in grey pig-iron, and in the "kish" of iron furnaces: it is also produced artificially on a large scale, together with .carborundum, in the electric furnace (see below).
Acheson, in 1896, patented an application of his, carborundum process to graphite manufacture, and in 1899 the International Acheson Graphite Co.
It combines directly with silicon, at the temperature of the electric furnace, yielding carborundum, SiC; and H.
Engineers and other craftsmen make extensive use of abrasion, effected by the aid of such abrasives as emery and carborundum, in shaping, finishing and polishing their work.
The production of aluminium in Switzerland and Scotland, carborundum and calcium carbide in the United States, and soda by the Castner-Kellner process, began to be conducted on an immense scale.