_Electric Welding_ was invented by Prof. Elihu Thomson, of Lynn, Mass., and patented by him August 10, 1886, No. 347,140-42, and July 18, 1893, No. 501,546. It is useful for the making of chains, tools, carriage axles, joining shafting, wires, and pipes, mending bands, tires, hoops, and lengthening and shortening bolts, bars, etc. For electric welding a current of great volume or quantity, and very low electro-motive force, is required. Thus a current of from one to two volts, and one to several thousand amperes, is best suited. Referring to Fig. 67, the current from the dynamo is conducted to one binding post of the commutator 3, which is arranged to send the current through one-sixth, one-third or one-half of the primary wire P of a transformer or induction coil. The other binding post of the commutator 3 extends to one terminal of an isolated primary coil 4, and the other terminal of this coil connects with the dynamo. The coil 4 is provided with a switch to regulate the amount of current. The rods to be welded are placed in clamps C C′, C being connected with one terminal of the secondary conductor S, and the movable clamp C′ with the other. When the current is turned on C′ is moved so as to project one of the surfaces to be welded against the other, and as they come in contact they heat and fuse together, as shown at W. Larger apparatus has been devised to weld railroad joints on the roadbed, and for other applications. [Illustration: FIG. 68.--GENERATION OF ELECTRICITY BY COMBUSTION.] _The generation of electricity_ for commercial purposes is almost entirely dependent upon the dynamo, as this is cheaper than the voltaic battery. The dynamo, however, must be energized by a steam engine. The direct production of electric energy by the combustion of coal would be the ideal method. A process invented by Edison (Pat. No. 490,953, Jan. 31, 1893), is interesting as an effort in this direction, and is presented in Fig. 68. A carbon cylinder D is suspended in an air-tight vessel B, and is surrounded by oxide of iron F, the whole being placed above a furnace. The temperature being raised to a point where the carbon will be attacked by the oxygen, carbonic oxide and carbonic acid will be formed, which are exhausted by the suction fan E. A constant current of electricity is given off from the two electrodes through the wires, the metallic oxide being reduced and the carbon consumed. [Illustration: FIG. 69.--RUDDER AND MOTOR OF TROUVÉ’S ELECTRIC BOAT, 1881.]