WELDING (from weld, variant of well, in lluenccd by Dan. nelde, to boil, front AS. weal 011G. wallah, to boil, well up). The process by which certain substances, such as iron and platinum, horn and tortoise-shell, are united when in a softened state due to being heated. The welding of iron consists in heating the two pieces until they attain a plastic and slightly viscous condition, placing them in contact with each other, and hammering or pressing the soft parts together. The process with other sub stances is essentially the same. In recent years use has been made of the electric arc for welding iron, and more particularly for welding street railway rails after they are laid in the track. For this latter purpose the electric welding equipment, consisting usually of four cars, is run out on the track to he welded. The first car carries an air compressor and sand blast, by which the rails at the joints are thoroughly cleaned after removing the splice plates. The
second car carries the 'welder,' which is simply a large transformer with jaws that can be clamped neon the rail. By means of these jaws a bar of steel is clamped to the rail on each side with a pressure of about 1400 pounds. Current is then turned on and continued about two minutes. This heats the splice bars and rails sufficiently to cause the metals to flow and the bars and rails to unite firmly. Current is then turned off and the clamping pressure is increased to about 35 tons and continued until the joint cools. This process is repeated twice for each joint. The third and fourth ears contain ap paratus for steadying the voltage of the current taken from the trolley wire and for transform ing it from about 500 volts direct to the welder transformer, which in turn transforms it to from two to four volts and 30,000 to 50,000 amperes.