When welding with direct current it is important that the work and electrode are connected to the correct terminals of the power supply. In the early days of arc welding, bare electrodes were almost always used with the holders connected to the negative terminal and the work connected to the positive terminal of the power supply. This was known as straight polarity but the standard term is now direct current electrode negative (DCEN).
Heavily coated electrodes may be connected and operated on either polarity, but most types are connected to the positive terminal with work connected to the negative terminal. This was formerly known as reverse polarity, but now the standard term is direct current electrode positive (DCEP).
Normally, about two thirds of the arc heat is developed at the positive terminal and one third at the negative terminal. The heavily coated electrodes used in shielded metal arc welding (SMAW) require that the most heat be developed at the positive wire electrode, where it is needed to melt the wire. For gas tungsten arc welding (GTAW), the most heat is developed at the positive workpiece, where it is needed to melt the metal in the joint. The negative tungsten electrode does not melt, even though it is ground to a fine point. When welding aluminum using DCEP, which is required to clean the aluminum oxide from the surface, a much larger diameter electrode with a hemispherical tip is used to help dissipate the extra heat and prevent the electrode from melting.
When alternating current is used, there is little difference in the heat developed at either pole because the polarity changes every half cycle. Alternating current is advantageous for welding aluminum because it provides a cleaning action with less heat developed at the electrode.