When an electric current encounters resistance in its circuit a portion of its energy is converted into heat. If an electric current flows across the junction of two rods placed in mutual contact, more or less heat will be generated at this joint. If the joint does not present great resistance, a weak current will traverse it, and a large quantity of current will be required to generate enough heat to weld the rods together. If, however, the joint presents moderate resistance, due to the rod ends being more or less separated by air space, or by scale, sand, or other non-conductor, a stronger current is required to flow across the resistance, and a much less quantity of current will produce a welding heat.

Application is made of these conditions in electric welding operations. The first-named condition, that requiring the lesser strength of current is known as the resistance system, and the second condition, requiring strong enough current to produce an electric arc, is called the arc system. In both systems the strength and quantity of current for welding depends upon the size of the weld and the time consumed in making it. The ordinary lighting circuit is strong enough for many needs in welding.