# Why does this home experiment show no trace of a magnetic field?

The photos below show what I did. Where are the fields? The first photo shows welding cables over paper with iron filings. 55 amps DC and 70 amps DC produced only faint hints of a field. The second photo shows 60 Hz over paper with iron filings (notice I separated the conductors so they wouldn't cancel each other). I then ran a small motor with the ac and no noticeable field was created in the filings. Then I remembered that the magnetic field is perpendicular to the electric field so I ran the wire perpendicular to the paper and still no field. The very last photo shows the hint of a field with the welding cables.

In summary here's what I did: I tried DC with a lot of current. I tried AC with a motor as the draw. I tried perpendicular and parallel, and basically got nothing. Besides rolling his eyes at me, what would Maxwell say?

• @Lambda One obvious possible culprit for this is that the cables include both out and back leads, so that the net current over each wire is essentially zero. What wires did you use? Have you got access to their cross-section? What exactly did you use as a current source, and how did you connect the cables to it? Mar 16, 2017 at 23:18
• To complement Emilio's comment, are the cables coaxial or carrying current in both direction? In other words, are there multiple wires in the cable that carry current in both directions? To see a magnetic field, each cable should have one conductor and carry current in one direction. Mar 17, 2017 at 0:18
• The welding cables are single conductor one positive the other neg. The source for the welder is a 110 outlet. It's an inverter welder. The AC portion, with the motor, was regular zip cord separated. It too had a 110 source. Maybe the issue is that the welder is pulsing DC rather than regular DC.??? The inverter converts the AC to DC but it must be pulsating DC right?? Mar 17, 2017 at 0:23

The magnetic effect of a direct current is very weak. At 1cm from a wire carrying 70 A the magnetic field is about 14 gauss. This is significantly stronger than the Earth's magnetic field (0.5 gauss) but still weaker than a fridge magnet (about 100 gauss). Does a fridge magnet produce a pattern in the iron filings? Getting a pattern with a strong magnet is difficult enough.

You will not see any effect in the setup of figs 1, 2 & 4 because the magnetic field is vertical at the paper. It will not push iron filings either towards or away from the wire, nor clump them in lines parallel to the wire.

You would only see an effect in fig 3. But all that will happen in this case is that the filings will each turn parallel to the field - you have to look very closely to see the effect. The filings should be longer in one direction, otherwise they will not turn and you will not notice they have turned. If the particles are round they will not turn in the magnetic field. If the filings are close enough together a ring of filings might join up end to end around the wire. Your filings are much too far apart for this too happen. They will not move closer to the wire.

The magnetic force may be weaker than the static friction force, keeping the filings from moving at all.

You will not see any effect with AC, because the magnetic field changes direction 60 times per second. The iron filings do not have time to react before the direction of the force on them reverses.

• @Lambda To obtain concentric circles in figure 3 (which is how the filings should arrange themselves around the conductor in an ideal case) try tapping the paper. You should something which resembles concentric circles approximately. Mar 17, 2017 at 2:07

Update on this project. I finally figured out a way to get a sense of the magnetic field generated from the welding cables. I suspended two small super magnets from a thread and then welded while the iPhone camera was recording. The magnets jumped and danced quite a bit. Even at a distance of 6 inches from the cables there was a clear pronounced movement of the magnets. Now here's what really surprised me, even at a distance of one meter there was still a noticeable effect. At this distance the magnets were on a separate table, and I took great care to rule out other sources. I would post the video, but this site doesn't support video so here's a pic. Thanks to all who responded to this post. Your info and insight was very helpful. Any insights on the one meter effect?

• So what is the shape of the magnetic field of a DC wire? May 18, 2023 at 9:13