(1) In a spark plug one electrode (the centre electrode) is the centre rod passing through the insulator. The side electrode is a rod welded to the shell of the spark plug. The distance between them is the spark gap.

(2) In welding it is the metal rod that is used in arc welding. A substance which brings electricity up to the point where the arc is to be formed; in other words it is the material immediately adjacent to the arc proper and the one which carries the current to this point. In electric arc welding the electrode is usually melted and becomes a part of the weld.