 The type of current used plays an important role in arc welding . <unk> electrode processes such as shielded metal arc welding and gas metal arc welding generally use direct current , but the electrode can be charged either positively or negatively . In welding , the positively charged anode will have a greater heat concentration , and as a result , changing the polarity of the electrode affects weld properties . If the electrode is positively charged , the base metal will be hotter , increasing weld penetration and welding speed . Alternatively , a negatively charged electrode results in more shallow welds . <unk> electrode processes , such as gas tungsten arc welding , can use either type of direct current , as well as alternating current . However , with direct current , because the electrode only creates the arc and does not provide filler material , a positively charged electrode causes shallow welds , while a negatively charged electrode makes deeper welds . Alternating current rapidly moves between these two , resulting in medium @-@ penetration welds . One disadvantage of AC , the fact that the arc must be re @-@ ignited after every zero crossing , has been addressed with the invention of special power units that produce a square wave pattern instead of the normal sine wave , making rapid zero crossings possible and minimizing the effects of the problem . 
