Amperage & Polarity: The Two Dials
Amperage sets how much heat the arc delivers; polarity sets where that heat concentrates — electrode or workpiece. · 10 min
Every welding machine, however crowded its front panel looks, is asking you two questions. How much heat do you want? That is amperage. Where do you want it concentrated? That is polarity. Folio 3 measured the arc at a couple of thousand watts; this folio is about metering and aiming them. Before any table appears, put your instinct on record: how much current does a common rod actually take?
Guess before you learn
You are running a 1/8-inch E7018 rod on quarter-inch steel. What amperage do you dial in?
About 125 amps. There is a rule of thumb hiding in the rod itself: roughly one amp per thousandth of an inch of electrode diameter. An eighth of an inch is 125 thousandths — so start near 125 A, then adjust by what the puddle shows you.
9–12
3–5
The amperage dial works like a stove dial: a thin pan needs low heat or it scorches, and a thick pot needs high heat or nothing cooks. Thin metal, low amps; thick metal, high amps.
The polarity switch chooses which way the electricity travels around the loop. One direction pushes the heat deep into the metal; the other direction keeps it shallower and melts the rod faster. Same amount of heat — different place.
6–8
Amperage is the amount of electric current — how much charge flows each second. More amps mean more energy per second at the arc: a hotter, wider, deeper puddle. The starting rule: about one amp per thousandth of an inch of rod diameter, so a 1/8-inch rod (125 thousandths) starts near 125 A. Thicker metal calls for bigger rods and more amps; thinner metal, the reverse.
Polarity is the direction of that current. DCEP — electrode positive, also called reverse polarity — drives heat deep into the joint; most stick rods run here. DCEN — electrode negative, straight polarity — melts the rod faster and penetrates less, which suits thin sheet. AC flips direction 120 times a second: penetration in between, and it resists arc blow, a magnetic push that can make a DC arc wander.
9–12
The machine is a constant-current source: the amperage you set is what flows, while voltage floats with arc length. Power is volts times amps, so amperage is the lever on total heat. Too little for the rod and metal: the arc stalls, the rod sticks, and the bead humps up without biting in. Too much: violent spatter, undercut along the edges, burn-through on thin stock — the evidence trail of folio 11.
Polarity is geometry. In DCEP, electrons stream from workpiece to electrode while heavier positive ions bombard the work; with stick electrodes the net effect is deeper penetration. In DCEN the traffic reverses: faster rod melt-off, shallower bite. AC reverses 120 times each second at 60 Hz, averaging the two; because its magnetic field keeps reversing, it resists arc blow, the magnetic deflection that troubles DC arcs in corners and near the work clamp.
K–2
The welding machine has a heat dial. Turn it up and the arc melts faster and deeper. Turn it down and it melts less. Thick metal needs more. Thin metal needs less.
Electricity also flows in a direction — one way, or the other way. A switch picks the direction. The direction changes where most of the heat lands.
Undergrad
Heat input formalizes the dial: Q = ηVI/v joules per unit length of travel, with arc efficiency η near 0.7 to 0.9 for SMAW. Amperage enters linearly, but so does travel speed: the same setting run slower deposits more heat per inch. Current density (amps per unit of core-wire cross-section) is the truer limit on a rod — exceed it and the electrode overheats, degrading the coating before the arc ever sees the excess.
Heat partition between electrode and workpiece depends on polarity and process, but not by one universal fraction. In gas tungsten arc work, DCEN sends most heat into the work — which is how the tungsten survives. In SMAW, DCEP yields the deeper penetration, driven by ion bombardment, arc force, and coating chemistry. The honest statement: polarity redistributes the arc's energy budget, and the split is process-specific — measured, not assumed.
Postgrad
The classroom fraction — two-thirds of arc heat at the positive pole — deserves a footnote: calorimetric measurements scatter the anode/cathode partition widely across process, shielding gas, and current, because cathode-fall, anode-fall, and column contributions scale differently. What survives scrutiny is that partition is polarity- and process-dependent, and that SMAW's DCEP penetration advantage is a composite of ion momentum, arc pressure, and flux chemistry rather than any single mechanism.
Arc blow is the Lorentz story: the column is a flexible conductor deflected by asymmetric magnetic fields near plate edges and the ground-return path — mitigated by AC (field reversal outpaces the column's response), by relocating the clamp, or by welding toward the ground connection. Modern inverter AC squares the waveform to minimize dwell at the zero crossing, where the arc must re-ignite — an aside that matters most in the aluminum-TIG territory beyond this course.
polarity
Which direction direct current flows around the welding circuit. Electrode positive (DCEP) and electrode negative (DCEN) pass the same amperage through the arc — what changes is where the heat concentrates.
Amperage first. It rises with metal thickness — but not in the shape most people expect. For mild steel from thin sheet up to 3/8 inch, sketch how much amperage a single pass wants. Draw the shape before the numbers arrive.
Note
Every settings table is a starting point, not a verdict. The bead itself is the feedback — folio 8 teaches you to read it, and folio 11 to read its failures.
Now the second dial. A DC machine pushes current around the loop in one direction, and you choose which. Clamp the electrode holder's lead to the positive terminal and you are welding DCEP — electrode positive, which older welders call reverse polarity. Most stick rods, E7018 included, run here, and it drives penetration deep into the joint. Swap the leads and you have DCEN — electrode negative, straight polarity: the rod melts off faster, the arc bites shallower, and thin sheet stops burning through. AC removes the choice by reversing direction 120 times a second — and it has one standing virtue: arc blow, the magnetic wandering that can push a DC arc sideways in a corner, largely leaves AC alone.
Why is this true?
Why can the same 110 amps dig deep on one polarity and stay shallow on the other?
Because amperage fixes how much energy arrives each second, not where it lands. Reversing the current reverses which way electrons and ions travel across the arc, which shifts where their energy is deposited — the quantity is unchanged; the placement moves.
Set the machine for 3/16-inch steel with 1/8-inch E7018 — the steps fade as you master them
1/8 in = 125 thousandths → start near 125 A
E7018 at 1/8 in runs 110–165 A — 125 A sits comfortably inside
DCEP — the electrode lead on the positive terminal
Run a bead on scrap: rod sticking → up about 10 A; harsh spatter and undercut → down about 10 A
Unit I is complete: you can dress for the arc, manage its fume and fire, explain what it is, and now meter and aim its heat. What remains before striking one is hardware — the machine itself, its two heavy leads, and the clamp that closes the circuit. That is folio 5, and it opens the stick-welding unit proper.
Practice — new ink and old, interleaved
1.Mid-job, you notice the fume plume rising straight past your nose. The fix is —
2.In one sentence: why does lifting the rod far from the work kill the arc?
3.Your DC arc keeps wandering magnetically — arc blow — in a tight corner. Which change attacks the cause?
4.Rule of thumb: starting amperage for a 3/32-inch rod (94 thousandths)?
5.Why do cuffed pants and open shirt pockets matter to a welder?
6.Order these jobs from lowest amperage to highest.
- Thin sheet with a 1/16-inch rod
- 1/8-inch plate with 3/32-inch E7018
- 1/4-inch plate with 1/8-inch E7018
- 3/8-inch plate, 5/32-inch E7018 fill passes
7.About how hot does the arc's plasma column run?
8.Stick welding at 120 A — what suggested shade number?
9.About how far can welding sparks travel from the arc?