Using direct current, the tungsten electrode may be connected to either the negative or the positive terminal of the supply of power. In almost all cases, electrode negative is chosen. With that polarity, electrons flow from the electrode to the work and positive ions are transferred from the work to the electrode and the connection is known as DCEN (straight polarity). When the electrode is positive, the directions of electron and positive ion flow are reversed, the connection is known as DCEP (reverse polarity). With DCEN and a t hermionic electrode such as tungsten, around 70 % of the heat is generated at t he anode and 30 % at the cathode. Since DCEN produces the greatest amount of heat at the workpiece, to a given welding current, DCEN will provide deeper weld penetration than DCEP. DCEN is t he most common configuration used in TIG welding and is used with argon & helium or a mixture of the two to weld most metals.
When the tungsten electrode is connected to the positive terminal (DCEP), a cathodic cleaning action is created at the workpiece surface. This action occurs with most of the metals but is most important when welding aluminium and magnesium because it removes the refractory oxide surface that inhibits wetting of the weldment by the weld metal. Taken DCEN, in which the electrode tip is cooled by the evaporation of electrons, when the electrode is used as the positive pole, its tip is heated by the bombardment of electrons as well as by its resistance to their passage through the electrode.Therefore, to reduce resistance heating and increase thermal conduction into the electrode collet, a large diameter electrode is required for a given welding current when reverse polarity is taken. The current carrying capacity of an electrode connected to the positive terminal is approximately one tenth that of an electrode connected to the negative terminal. DCEP is generally limited to welding thin sheet metal.