Electric arc welding consists in joining two pieces of metal by filling in the cracks with molten metal. This is done by attaching or laying upon the steel or metal to be welded one wire of the current, and attaching the other wire to a piece of carbon or metal which the welder holds. The current passes from the piece which is to be welded to the electrode which the operator holds. Great heat is generated and consequently the electric arc is one of the most efficient methods of producing welding heat.