The generators are specially constructed with stationary alternating current coils and rotating exciting coils, an arrangement which is best adapted to withstand the unavoidable current impulses accompanying the welding process. The voltage and number of periods is chosen with regard to the requirements of the welding machine it works, and vary between 100 and 500 watts at fifty periods a second. The generator should be chosen large enough, so that the continual change from full load to no load will not damage the machine. If several welding machines are worked from one generator, the total of the power required by each single machine will be sufficient for the generator, as the average power required for an aggregate of welding machines is essentially less than the total of each single power.

The welding machines are generally delivered mounted and ready for working, requiring only the electrical connections in as simple a manner as that for an incandescent lamp.

Nearly all of the metals, even those like antimony and bismuth, which are brittle and crystalline, may be joined together by electric welding, and many different metals and alloys joined one to another. In some cases, as with high carbon steels, a flux, such as borax, is employed to facilitate union at temperatures not high enough to burn or destroy the texture of the metal. Mild steel and iron welds are usually made, as in ordinary forges, at welding heat, or that which melts or fluxes the ordinary black oxide scale upon the metal. The heating effect of the electric current is so perfectly adjusted by regulating appliances that most of the metals formerly regarded as unweldable yield good results. Even leaden pieces, such, for example, as sections of lead pipe, may be joined together with great ease.

The Electric Welding Company, Limited, sole owners of the Thomson patents in the United Kingdom, have given the following information as to their standard types of welders: -

The above welders are in some cases fitted with detachable clamps, so that other kinds of work may be welded by using different clamps. Many other types of welders are also built for special purposes.

In the earlier electric welding systems the operations of clamping the pieces in place, applying and cutting off the electric current and exerting mechanical pressure, were usually manually controlled. Machines more or less automatic are now frequently employed.

In recent types adopted for rapid repetition of work upon identical pieces the action is entirely automatic ; the machine runs continually and its sequence of actions is definitely determined by its construction. These machines are power-driven, movements being imparted for clamping the pieces as they are fed to the machine, for closing the current switch, for exerting pressure to complete the weld, for cutting off the current, and for releasing the pieces from the clamps after the operation. In wire fence and chain machines, for instance, the stock is itself fed automatically and the welding continued until the machine is stopped or the material exhausted.

The Thomson welding transformer is a construction like a lighting transformer, in which the usual secondary circuit of numerous turns is replaced by a very massive conductor •constituting ordinarily only a single turn around the iron magnetic core. The primary or inducing circuit is similar to that of the ordinary transformer for alternating current, and it is supplied from alternating current dynamos on lines as usual in such work. The secondary conductor is unique in character, being often a bar or casting of many square inches of section of copper of short length. The circuit of this single turn secondary is completed only by the meeting ends of the work pieces in the clamps. It will thus be evident that the chief resistance or opposition to the flow of the low-voltage current in the single secondary turn will be at the proposed joint or weld between the clamps. Here it is that the transformed energy is for the most part given out as heat, the section of metal which can be welded depending upon the scale of the apparatus used and the energy of the primary source which is available.

The welding transformer has found convenient application in the heating of metal pieces for forging, bending, shaping, brazing or the like. It has also in the Lemp process been divested of its welding clamps and applied to the local annealing of the hardened face of armour plates, so as to facilitate drilling and tapping, or cutting into desired shapes.

The welds made by the Thomson process are usually butt welds, though lap welds are also made with equal facility. In butt welding there is, of course, an upset, burr, or extrusion of metal at the joint. In many cases this is not removed, and it renders the joint stronger than other adjacent sections; oftentimes the joint is pressed or forged while still hot, so as to remove the burr at the joint. In other cases the joint is-finished by filing or grinding.

The welding clamps are modified in form and disposition to suit the shape and size of the pieces to be held, and the pressure used to effect the weld is either manually applied by levers or is obtained from a strained spring, or again, in large works, by hydraulic means under control of suitable valves.

The energy required to effect electric welds naturally varies with the size of the pieces and with the material. It also depends upon the time consumed in the work, which time may be made shorter or longer even with exactly similar pieces.

The following table gives the results of some tests made upon different sections of iron, mild steel, brass and copper in the form of bars. The figures are stated to be only approximate. In general, working at a greater rapidity would lessen the total power used, but require larger apparatus for the increased output required during the welding.