in diminishing the formation of surface scale. Tests show that the electric weld is much stronger than that made in the ordinary way in a forge, and, indeed, is in some cases stronger than other parts of the bar.
The machines designed by Prof. Thomson for carrying out this method of welding are extremely simple, the mechanical part consisting essentially of one or more pairs of clamps to hold the pieces to be united, and means for pressing them together while in a heated condition. In operating the machines the current is turned on by the workman by means of a switch; but Prof. Thomson has taken advantage of the movement of the pieces toward each other while the weld is being made to break the circuit, thus rendering the operation automatic and insuring the equal heating of the welded pieces. In machines for wire and small rod the welded wires and rods are pressed together by means of springs, but in those for larger work the necessary pressure is applied by hydraulic apparatus. The necessity for this will be appreciated when it is stated that the pressure requisite for steel is 1,800 pounds to the square inch, that for iron 1,200 pounds, and for copper 600 pounds.
Electrically the apparatus is as simple as it is mechanically. The alternating current, which has shown itself so flexible in the hands of the engineer in other departments of electrical work, is here called into requisition. Through the medium of converters the high potential machine current is transformed into others of great volume and low voltage suitable for this class of work. Currents of this character are rendered necessary by reason of the fact that all metals are very good conductors of electricity, and can therefore be heated only by currents of great amount. These currents range, in fact, from a few hundred amperes to eight and ten thousand. The voltage, however, is very low, rarely being more than four or five volts, and in large and heavy work sometimes not more than a single volt. On account of this very low electrical pressure all danger from the current is eliminated and the apparatus may be handled with the same freedom as any ordinary metal-working machine. In the distribution of the electrical appliances the current is usually generated by a machine conveniently located with reference to the source of power, and the current carried by wires to the welders, where the transformation takes place, each welder being provided with its own converter, proportioned so as to supply the character of current best suited to the special work of the machine. The current is under perfect control by means of regulating devices operated by the workman, the usual device employed being a reactive coil. The range of work possible with this method of welding is very great. It not only may be used in forming all ordinary welds with iron and steel, but