The use of the electric arc as the source of heat for joining and melting metals is one of the oldest applications of electricity and yet it was not developed to a commercially practical point until within a comparatively few years. In 1786, Martinus van Marum published a book, in Leipzig, describing some early electrical experiments, and, in this book, he gave an exhaustive treatise on the melting of metals by means of the electric current. In 1810, Sir Humphrey Davy, the versatile English scientist, described some experiments made in London with various metallic bodies, and, in 1815, J. G. Children of Londen described a process for welding iron wire with the electric arc obtained from batteries. From then until now, the development of the art of electric welding has progressed steadily, so that today it stands as the most universal of all welding systems and is being adopted rapidly for all classes of metal manufacture and repair. The men who have done most to perfect electric-arc-welding processes are. De Meritens, Bernardos, Olszewsky, Coffin, Zerener, Slavianoff, Howard, and a few others of lesser importance, and further reference to the processes developed by some of them will be made.
Fig. 51. Calking a Riveted Seam.
The electric arc has probably been given more careful study and investigation than any other electrical phenomenon and yet there is comparatively little exact knowledge available regarding some of its most important characteristics. The exact temperature of an arc has never been determined although the most refractory substances may be melted in its vapor and, since the vapor is so hot, it is a very efficient source of heat. The temperature has been variously estimated at from 2000 degrees centigrade by some scientists up to 6000 degrees centigrade by others, but the temperature generally accepted as correct is about 4000 degrees centigrade. As long ago as the year 1840, Grove discovered that the current flows more easily from metal to carbon than in the reverse direction and that the current through an arc is greater, when passing from an easily oxidized metal to one that is not, than when flowing in the reverse direction.
The explanation of this is comparatively simple. The conductivity of an arc depends largely upon the kind of vapor in the arc and, to some extent, upon the ease with which the cathode (negative electrode) can be kept at a high temperature. If the anode (positive electrode) gives off a conducting vapor when heated, this vapor will help the conductivity of the arc. In the arc-welding systems in commercial use today, the arc is drawn between metal and carbon, or between metal and metal and, since the positive electrode or terminal of an arc reaches a higher temperature than the negative electrode,, it is more efficient to use the article worked upon as the positive electrode of the are. Since iron is more easily vaporized than carbon, the current flows more easily from iron to carbon than the reverse, because there is more iron vapor than carbon vapor in the arc. This is proved by the fact that it requires more voltage to send a given current through an arc between carbon electrodes than between iron ones. It is also important that the negative electrode be kept at a high temperature and the usual practice of having the negative electrode small (due to the use of a wire or carbon pencil) makes this easily possible.
Courtesy of Westinghouse Electric and Manufacturing Company.