There are two kinds of electrical furnaces, as there are two kinds of electric lights—arc and incandescent. Moissan has used the arc furnace in all of his experiments, but Mr. Acheson's furnaces follow rather the principle of the incandescent lamp. "The incandescent light," said Mr. Fitzgerald, "is produced by the resistance of a platinum wire or a carbon filament to the passage of a current of electricity. Both light and heat are given off. In our furnace, the heat is produced by the resistance of a solid cylinder or core of pulverised coke to the passage of a strong current of electricity. When the core becomes white hot it causes the materials surrounding it to unite chemically, producing the carborundum crystals."
The materials used are of the commonest—pure white sand, coke, sawdust, and salt. The sand and coke are mixed in the proportions of sixty to forty, the sawdust is added to keep the mixture loose and open, and the salt to assist the chemical combination of the ingredients. The furnace is half filled with this mixture, and then the core of coke, twenty-one inches in diameter, is carefully moulded in place. This core is sixteen feet long, reaching the length of the furnace, and connecting at each end with an immense carbon terminal, consisting of no fewer than twenty-five rods of carbon, each four inches square and nearly three feet long. These terminals carry the current into the core from huge insulated copper bars connected from above. When the core is complete, more of the carborundum mixture is shovelled in and tramped down until the furnace is heaping full.
Everything is now ready for the electric current. The wires from the Niagara Falls power-plant come through an adjoining building, where one is confronted, upon entering, with this suggestive sign:
DANGER
2,200 Volts.
Tesla produces immensely higher voltages than this for laboratory experiments, but there are few more powerful currents in use in this country for practical purposes. Only about 2,000 volts are required for executing criminals under the electric method employed in New York; 400 volts will run a trolley-car. It is hardly comfortable to know that a single touch of one of the wires or switches in this room means almost certain death. Mr. Fitzgerald gave me a vivid demonstration of the terrific destructive force of the Niagara Falls current. He showed me how the circuit was broken. For ordinary currents, the breaking of a circuit simply means a twist of the wrist and the opening of a brass switch. Here, however, the current is carried into a huge iron tank full of salt water. The attendant, pulling on a rope, lifts an iron plate from the tank. The moment it leaves the water, there follow a rumbling crash like a thunder-clap, a blinding burst of flame, and thick clouds of steam and spray. The sight and sound of it make you feel delicate about interfering with a 2,200-volt current.
The Interior of a Furnace as it Appears after the Carborundum has been Taken Out.
This current is, indeed, too strong in voltage for the furnaces, and it is cut down, by means of what were until recently the largest transformers in the world, to about 100 volts, or one-fourth the pressure used on the average trolley line. It is now, however, a current of great intensity—7,500 ampères, as compared with the one-half ampère used in an incandescent lamp; and it requires eight square inches of copper and 400 square inches of carbon to carry it.
Within the furnace, when the current is turned on, a thousand horse-power of energy is continuously transformed into heat. Think of it! Is it any wonder that the temperature goes up? And this is continued for thirty-six hours steadily, until 36,000 "horse-power hours" are used up and 7,000 pounds of the crystals have been formed. Remembering that 36,000 horse-power hours, when converted into heat, will raise 72,000 gallons of water to the boiling point, or will bring 350 tons of iron up to a red heat, one can at least have a sort of idea of the heat evolved in a carborundum furnace.