1.3 The Welding Arc
Most metals and alloys conduct electricity at room temperature due to the presence of free electrons.
A considerable amount of heat can be produced from the flow of the current in a circuit. Typical
examples of this heating effect, also called resistance heating, are tungsten filament bulbs and heating
coils in ovens. In comparison, gases like oxygen, nitrogen, carbon dioxide, etc. do not conduct any
electricity at room temperature. However, if sufficient energy is applied to a gas it also can become
conductive. When sufficient voltage is applied to a gas it can be ionized – changed into positively
charged ions and negatively charged electrons. The electrons move in response to the applied voltage
to produce a current flow and this movement of electrons allows the initiation of an arc. The current
flow causes resistance heating in the gas which promotes further ionization and increased current flow.
As long as the voltage source is able to supply the necessary voltage and the current needed by the
arc, it can be sustained in a stable manner and used for welding applications.
Based on the above principle, a conventional arc is formed between two non-consumable electrodes in
a gas or vapour medium when an appropriate voltage, depending on the electrode material and gas
phase, is applied to the electrodes. As seen in Figure 1.1, one of the two electrodes forms a positive
terminal of the electrical circuit and is called the anode; the negative terminal of the circuit is called the
cathode. When an arc is created, electrons are evaporated from the cathode and transferred to the
anode through the ionized gas in between. Flow of electrons is the same thing as flow of current or
electricity.