Re: magnetism and arc welding

Hi Kyle!

You're right, the magnetic attraction is fairly weak. The 150 amperes can make a compass deflect, but it can't attract nails. It CAN twist small metal bits around. Try putting some steel powder on the floor under the welder's cable, and after you run the welder, you'll find that the powder has aligned itself perpendicular to the direction of the current. (In shops that have a lot of metal bits on the floor, you'll see this happen accidentally.)

To strongly attract iron, a magnetic field must be strong, but it also must be "radial" or splayed-out in shape (like the field near the ends of a bar magnet.) If the field lines are parallel instead, then the field will cause iron object to twist, but it won't attract them. The Earth's magnetic field does this: it can twist a compass needle, but since the flux lines are parallel, the entire compass doesn't go flying off to the north pole or the south pole of the Earth.

The field around a DC cable goes in a circle (like invisible donuts, with the cable going through the donut holes.) There IS a way to pick up iron objects with this field. Get a "U" shaped piece of iron, and wrap the welder cable around it. Run the current through the cable, then put the two ends of the "U" against an iron or steel object. You can probably pick up that object. The "U"-shaped iron has become a horseshoe magnet. The iron makes the magnetic field stronger, and it also gives it that "splayed out" shape near both ends of the "U", and this strongly attracts other pieces of iron.

When you're welding iron or steel, there should be a circular magnetic field inside the liquid metal under the arc. I think this field should squeeze itself together (like a shrinking donut). It should make the puddle of liquid metal hump upwards, but only very slightly. When the arc turns on and off, does the puddle of metal lift up and down a very tiny bit? (Hard to see this with welding goggles!)