Fig. 182 shows how the current from two strong cells can be used to heat a short length of very fine platinum or German-silver wire. The copper conductors attached to the cells do not offer very much resistance.
It will be seen from the above that in all electrical work the sizes of the wires used have to be such that they do not overheat. The coils of dynamos, motors, transformers, ampere-meters, etc., etc., become somewhat heated by the currents passing through them, great care being taken that they are properly designed and ventilated so that they will not burn out.
Fig. 183.
Fig. 184.
146. Electric Welding. Fig. 183 shows one form of electric welding machine. The principle involved in the art of electric welding is that of causing currents of electricity to pass through the abutting ends of the pieces of metal which are to be welded, thereby generating heat at the point of contact, which also becomes the point of greatest resistance, while at the same time mechanical pressure is applied to force the parts together. As the current heats the metal at the junction to the welding temperature, the pressure follows up the softening surface until a complete union or weld is effected; and, as the heat is first developed in the interior of the parts to be welded, the interior of the joint is as efficiently united as the visible exterior. With such a method and apparatus, it is found possible to accomplish not only the common kinds of welding of iron and steel, but also of metals which have heretofore resisted attempts at welding, and have had to be brazed or soldered.
Figs. 185 to 189.
The introduction of the electric transformer enables enormous currents to be so applied to the weld as to spend their energy just at the point where heating is required. They need, therefore, only to be applied for a few seconds, and the operation is completed before the heat generated at the weld has had time to escape by conduction to any other part.