The arc in welding is a sustained electrical discharge between an electrode and the metal workpiece. It creates an intensely hot column of ionized gas, called plasma, that reaches temperatures around 20,000°C at its center. This plasma is what melts the base metal and, in most processes, the electrode itself to form a weld joint.
How the Arc Forms
An arc starts when electrons escape from the electrode (the cathode in most setups) and accelerate across the gap toward the workpiece. That acceleration is driven by the voltage difference between the two. As these fast-moving electrons collide with gas molecules in the gap, they knock loose more electrons and create positively charged ions. This chain reaction transforms ordinary gas into plasma, a superheated, electrically conductive column that can carry large amounts of current.
Once this plasma column is established, current flows continuously. The gas stays ionized as long as enough energy is being fed in, which is why the arc sustains itself during welding. If you pull the electrode too far away, the voltage can’t push electrons across the wider gap and the arc extinguishes. Move it too close, and the electrode touches the workpiece in a short circuit.
Three Zones of the Arc
The arc isn’t uniform. It has three distinct regions, each with different temperatures and behaviors.
The cathode spot is where electrons leave the electrode surface. Energy is consumed here to free those electrons, so the cathode spot is actually the cooler end of the arc. The anode spot is where electrons slam into the opposite surface, converting their kinetic energy into heat. This makes the anode the hotter pole. Between these two spots sits the plasma column, the visible, glowing body of the arc. Temperatures are highest at the center of this column and drop toward the edges.
These temperature differences matter because they determine where heat concentrates. In processes using a non-consumable tungsten electrode with the electrode as the negative pole, most of the heat goes into the workpiece, producing deep penetration. Reversing the polarity shifts more heat onto the electrode, which is useful when you want to melt a consumable wire faster.
How Polarity Changes Heat Distribution
Welding machines let you choose polarity, and this choice directly controls where the arc’s energy goes. With the electrode set as negative (called DCEN or straight polarity), electrons travel from the electrode to the workpiece. They slam into the base metal at high speed, generating significant heat there. This produces deeper weld penetration. Meanwhile, the electrode stays relatively cool because freeing electrons from its surface actually absorbs energy, creating a cooling effect on materials like tungsten that release electrons easily.
With the electrode set as positive (DCEP or reverse polarity), the situation flips. Electrons now travel from the workpiece to the electrode, heating the electrode more. For consumable wire processes like MIG welding, this is an advantage: the extra electrode heating melts the wire efficiently, and the molten metal transfers across the arc to fuse with the base metal. DCEP also has a cleaning action on aluminum and other metals with stubborn oxide layers, because ions striking the workpiece surface break up those oxides.
How Molten Metal Crosses the Arc
In consumable electrode processes, the arc doesn’t just provide heat. It also moves molten metal from the wire tip to the weld pool. This transfer happens in several distinct ways depending on current, voltage, and shielding gas.
At low current settings, short-circuit transfer occurs. The wire tip physically touches the weld pool 20 to 200 times per second, depositing small amounts of metal with each contact. Surface tension pulls the droplet off the wire, and the arc reignites after each brief short circuit. This mode runs cool and works well for thin materials or out-of-position welding.
At higher voltages with carbon dioxide-rich shielding gas, globular transfer takes over. Large droplets, typically three times the wire diameter, form at the electrode tip and fall across the arc mainly under the force of gravity. The transfer rate drops to just a few droplets per second, making this mode less smooth and more prone to spatter.
At higher currents still, spray transfer produces a stream of tiny droplets that cross the arc in a steady, directed spray. This mode delivers high deposition rates and clean welds but generates considerable heat, limiting its use to thicker materials and flat or horizontal positions.
What the Shielding Gas Does
The gas surrounding the arc serves two purposes: it shields molten metal from atmospheric contamination, and it directly influences the arc’s behavior. Different gases require different amounts of energy to ionize, which changes how the arc starts, how stable it remains, and how much heat it delivers.
Argon ionizes easily, meaning less voltage is needed to strike and maintain the arc. This makes argon the go-to choice for stable, smooth arcs and easy arc starting. It also has a higher molecular weight, which concentrates its energy on the metal surface more effectively.
Helium requires significantly more energy to ionize, so arcs in helium run at higher voltages and deliver more total heat to the weld. This translates into deeper penetration and faster welding speeds on thick materials. The tradeoff is a less stable, harder-to-start arc. Many welders use argon-helium blends to get a balance of both characteristics.
Voltage, Current, and Arc Length
The arc’s electrical behavior follows a distinctive pattern. When the arc first strikes, voltage drops rapidly as the gas ionizes and becomes conductive. Current rises as voltage falls until the relationship becomes linear and predictable. From that point on, the arc behaves roughly according to Ohm’s Law: longer arcs have higher voltage but lower current, and shorter arcs have lower voltage but higher current.
Welding power sources are designed around this relationship. Manual processes like stick welding and TIG use constant-current power sources. These maintain a nearly steady current even as the welder’s hand moves and the arc length fluctuates. A typical constant-current source might allow only about 8 amps of variation for a 5-volt change in arc length at 150 amps. This stability prevents sudden current surges that would make the weld erratic.
Wire-feed processes like MIG use constant-voltage sources instead. Here, the power source holds voltage steady while current adjusts freely. If the wire tip gets closer to the workpiece (shortening the arc), current jumps, which melts the wire faster and restores the original arc length. If the arc gets longer, current drops, slowing the melt rate until the wire catches up. This self-adjusting mechanism keeps the arc stable without any input from the welder. Current swings of around 40 amps for a 5-volt change are typical in these systems.
Radiation Hazards From the Arc
The extreme temperatures in the plasma column produce intense light across the entire spectrum, from infrared through visible light to ultraviolet. The UV component is the most dangerous and the reason welding helmets use dark auto-darkening filters rather than simple tinted glass.
Welding arcs emit UV radiation across all three UV bands. UV-C (the shortest, most energetic wavelengths) and UV-B are the most harmful to skin and eyes. The most hazardous wavelength sits around 270 nanometers, deep in the UV-C range. Measured at just 500 millimeters from the arc, effective UV irradiance can reach up to 2.2 milliwatts per square centimeter, which is enough to cause eye damage (arc flash or “welder’s flash”) in seconds of unprotected exposure and sunburn-like skin damage within minutes.
The specific metals being welded change the radiation profile. Iron produces strong UV emissions in the 240 to 275 nanometer range, while nickel-containing electrodes add emissions around 280 nanometers. This means welding different alloys can change your UV exposure even if the process settings stay the same.