First, there is a drop in voltage over the electrode extension, that is the length of electrode between the
point of electrical contact with the contact tip, and its melting tip, also called cathode spot for the
current flow direction shown in the sketch. The magnitude of this voltage drop depends on the
electrode extension and the wire diameter as well as the current; a longer electrode extension, a
smaller wire diameter or higher current all increase the voltage drop over the electrode extension
length.
The voltage drop over the arc length, that is the distance between the cathode spot and the anode spot
(the molten weld pool surface in Figure 1.5) takes place in three steps. Right next to the anode and
cathode spots are small, thin, gaseous regions called the anode drop zone and cathode drop zone,
respectively, and over these zones there can be a significant drop in voltage, in the range of 1 to 12 V
depending on the electrode material.
In between the two drop zones, there is the arc column with a relatively small drop in voltage, of the
order of 1 to 2 V per centimetre length of the arc column. There is a jet-like flow of ionized gases in
the arc column that gives it some stiffness and force (resistance to deflection). This enables the welder
to manipulate the gun and direct the molten metal to be deposited at the desired location in the weld
joint. Shorter arcs have greater stiffness than longer arcs.
Arc length is a critical and controllable parameter, which is directly related to the arc voltage. Arc
voltage depends on the space between electrodes; electrode composition, diameter and extension;
shielding gas composition; metal thickness; joint design; welding position, etc. The voltage measured
at the power supply is greater than the arc voltage. Output voltage represents the sum of arc voltage
and the voltage drop in the remaining part of the electrical circuit. The longer the electrical cables the
greater will be the difference between the voltage read at the power supply gauge and the actual arc
voltage.