Arc mode is a type of electrical discharge where current flows through ionized gas (plasma) at high intensity and relatively low voltage. It occurs naturally in lightning and is harnessed deliberately in welding, lighting, spectroscopy, and other industrial processes. What distinguishes arc mode from other forms of electrical discharge is what happens at the electrode surface: the current density at each active spot on the cathode reaches extreme levels, on the order of 10¹² amps per square meter, creating a self-sustaining, intensely hot plasma channel.
How Arc Mode Differs From Glow Discharge
Electrical discharges in gas exist on a spectrum, and the two most commonly compared modes are glow discharge and arc discharge. Older textbooks defined them simply: arcs carry high current (well above 1 amp) at low voltage (under 50 volts), while glow discharges carry low current at high voltage, often several hundred volts. That description is useful but incomplete.
The real difference lies in how electrons leave the cathode. In a glow discharge, electrons are knocked loose one at a time when ions strike the cathode surface. This process is inefficient, which is why glow discharges need high voltage to sustain themselves. In arc mode, the voltage drop near the cathode is small, around 10 volts, but that’s enough to create an intense electric field at the surface. This field pulls electrons out of the metal directly, a process called field emission. Because electrons flow so freely, arc mode sustains itself at far lower voltages while carrying far more current.
The transition from glow to arc is often sudden. When arcing starts, the voltage drops sharply while current spikes. This is why unintended arcing in electrical equipment can be destructive: the system wasn’t designed for that much current in that small a space.
Temperatures Inside an Arc
Arc plasmas are extraordinarily hot. The core of an atmospheric-pressure arc can reach 20,000 to 30,000 Kelvin, hundreds of times hotter than a candle flame and roughly five times the temperature of the sun’s surface. At these temperatures, the plasma radiates across a huge range of the electromagnetic spectrum, from infrared through visible light and deep into ultraviolet. This intense heat is what makes arc mode so useful for cutting, welding, and vaporizing materials, and so dangerous without proper protection.
Arc Modes in Welding
In gas metal arc welding (commonly called MIG welding), “arc mode” usually refers to how molten metal transfers from the electrode wire to the workpiece. The three primary transfer modes behave very differently, and choosing the right one affects weld quality, spatter, and what positions you can weld in.
Short-Circuit Transfer
At lower currents and voltages, the electrode wire physically touches the molten weld pool between 20 and 200 times per second. Each contact creates a brief short circuit that deposits a small amount of metal. This mode works well for thin materials and out-of-position welding because heat input is low. The tradeoff is more spatter, since each short circuit flings off small droplets.
Globular Transfer
At moderate settings, large molten droplets form at the tip of the wire and fall into the weld pool mainly under gravity. This mode is generally the least desirable. Electromagnetic forces and vapor jets at the base of each droplet can push it sideways or even repel it from the arc entirely, creating poor stability and heavy spatter.
Spray Arc Transfer
At high currents and voltages, the wire melts into a stream of very fine droplets that are driven across the arc by strong electromagnetic forces. Spray transfer is essentially spatter-free and produces smooth, high-quality welds. It requires argon or argon-rich shielding gas, a small-diameter wire, and high wire feed speed. The high heat input makes it best suited for thicker materials welded in flat or horizontal positions.
Arc Mode in Chemical Analysis
Before modern instruments dominated analytical chemistry, direct-current arc excitation was the leading technique for identifying trace elements in solid samples, particularly in geochemistry. A DC arc between carbon electrodes generates temperatures around 4,000°C at the electrode tip, hot enough to destroy tough ceramic materials like silicon carbide and boron carbide and completely vaporize impurities from powdered samples. Each element in the vaporized material emits light at characteristic wavelengths, which a spectrometer reads like a fingerprint.
This approach remains a practical alternative to more complex modern methods because it requires no special sample preparation and no controlled gas atmospheres. It’s particularly useful for analyzing environmental sediment samples. The precision is moderate, around 15% relative, but the detection limits are low enough for many real-world applications.
UV Radiation Hazards From Arc Discharges
Any arc discharge produces ultraviolet radiation across nearly the full UV spectrum, from about 190 nanometers up through 400 nanometers. (Wavelengths below 190 nm are absorbed by oxygen in the air and don’t reach you.) When welding steel, most of this UV comes from the spectral lines of iron atoms in the plasma.
The health consequences are significant and well documented. Acute exposure causes photokeratitis, sometimes called “welder’s flash” or “arc eye,” an intensely painful inflammation of the cornea that produces a gritty foreign-body sensation, tearing, and sensitivity to light. A survey of 1,667 workers at 47 welding workplaces found that 86% had experienced these symptoms, and 45% reported recurring episodes at least once a month, even though most wore face shields. A separate survey found 92% of welders had suffered at least one flash burn, and 40% had experienced UV skin burns on the neck. Long-term exposure increases the risk of cataracts and skin cancer.
Proper protection means auto-darkening welding helmets rated for the specific arc process, UV-blocking safety glasses for nearby workers, and skin coverage for exposed areas like the neck and forearms. Bystanders within line of sight of a welding arc are also at risk, which is why welding curtains and screens are standard practice in shared workspaces.
Arc Mode in Radiation Therapy
In a completely different context, “arc mode” appears in cancer treatment. Volumetric modulated arc therapy (VMAT), introduced in 2007, delivers radiation to tumors while the machine’s arm rotates continuously around the patient in an arc. During this rotation, three variables change simultaneously: the speed of rotation, the shape of the radiation beam (controlled by dozens of small metal leaves that slide in and out), and the dose rate. This allows the treatment to concentrate radiation tightly on the tumor while minimizing exposure to surrounding healthy tissue. Compared to older fixed-angle techniques, arc-based delivery is faster and can produce more precisely shaped dose distributions.