WELDING, ELECTRIC. One of the oldest known facts in the working of iron is that portions. when softened or rendered plastic by heat, could, under suitable conditions, unite or weld together. Owing to this property, the earliest iron smelters were able to secure pieces of moderate size from the granules obtained in the reduction of the ores in their crude furnace operations, These were carried on on too small a scale to give rise to iron which had in it a portion of carbon conferring fusibility, or cast-iron. In general, softened materials, such as warm wax, piteh. or heated glass, possess the property of welding in an eminent degree. This is probably due to the existence of a comparative freedom of movement of the molecules of materials in a plastic condition, which allows the cohesive force to be exerted between or surfaces brought very near together, or into complete contact one with the other. or such operations of welding, the surfaces must be clean and free from interposed scale or dirt, or the conditions must be such that these latter arc expelled from the joint during the operation. With platinum or glass in the heated or softened state, the union takes place with great facility, owing to the non-oxidabilitv of the surfaces in contact, loot in the case of such a metal as iron, which forms a reale of black oxide when heated in the air, the tem perature for welding must either be so high that the oxide is liquefied and so made to exude from the joint or surfaces brought together ; or, for the same exudation or auto-cleansing to take place, a flux which dissolves and renders liquid such scale or oxide at a lower tempera ture is required. In still another extrusion of sufficient of the metal itself outwardly from the joining surfaces—the condition of absence of scale or oxide at the joint may be secured. The application of the heating effects of electrical currents, together with mechanical manipulation, marks a recent advance in the art of welding metals. The well
known ease with which electrical currents may be regulated or governed in their effects, con tributes greatly to the success of the operation.
The principles of the Thomson process of electric welding, which principles are, with some modifications, applied to the operations of electric forging and shaping, upsetting, riveting, etc., may be briefly stated as follows The pieces to be operated upon are held in suitable clamps or supports, and provision made for the passage of heavy currents of elec tricity at very low pressures or potentials through the joint or from piece to piece. The cur rent usually enters by the holding-clamps, though sometimes other means than the clamps are used to pass current into the pieces. Indeed, in some eases no clamps are used, but merely contact surfaces bearing on the work-pieces at or near the joint. Various modifi cations are made in the devices employed so as to suit the character of the work itself. The result of passing a heavy current through the metal of the joint is a localization of the heat ing effect of the currents to the joint itself, or, more correctly, to the metal at the joint and at a small distance each side of it. During the passage of current, the pieces are pressed together in firm contact, and since there is no arc, the heating occurs by the resistance of the solid metal, and not by that of any air or gas in a space between them. Neither does the heating altogether depend on the fact that the meeting portions do not fit perfectly, such imperfect fit giving increased resistance at the joint, for a solid bar joining the clamps would be heated between such clamps, though its resistance is not increased by the existence of any break or partial fit of surfaces in contact., The heat developed in any portion of an electric circuit depends upon the resistance which it offers to the current and upon the amount of current passing. It. is also in proportion to the square of the strength of that current.