40 THE POPULAR SCIENCE MONTHLY. 



in diminisliing the formation of surface scale. Tests sliow that 

 the electric weld is much stronger than that made in the ordi- 

 nary 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 cur- 

 rent 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 GOO 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 some- 

 times 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 ordi- 

 nary metal- working machine. In the distribution of the electrical 

 appliances the current is usually generated by a machine conven- 

 iently located with reference to the source of power, and the cur- 

 rent 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 con- 

 trol 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 



