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I understand a clamp meter works by measuring the magnetic field generated by current flowing through a wire.

It's well known that clamping 2 wires carrying the same current in opposite directions will show a null reading because one magnetic field is cancelling the other.

What I find surprising though is that if the second wire carrying the opposite current is placed immediately outside the clamp, it has no measurable effect. I was expecting its magnetic field to couple with the clamp to some degree but that seems not to be the case. Why?

Example

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  • \$\begingroup\$ Read this question and answer: electronics.stackexchange.com/questions/341156/… \$\endgroup\$ – Bimpelrekkie Jun 13 at 14:38
  • \$\begingroup\$ It does have a measurable effect. I have to be careful when I'm using my clamp around other magnetic fields such as those from a nearby transformer or coil. It introduces offsets and such and I can see the offset increase and decrease as I move the probe closer to the transformer or turn the current up. \$\endgroup\$ – DKNguyen Jun 13 at 19:29
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Imagine that the return conductor for the red-wire (the blue-wire in your picture) is a million miles away. You would probably agree that it has negligible effect.

Now as the blue return wire is brought closer, the localized magnetic field it produces helps the red wire's magnetic field (on the left) but it also produces an opposing magnetic field passing through the "ring" on the right.

The two opposing fields always cancel to produce the same net effect: -

enter image description here

The magnetic field from the red-wire not drawn for reasons of clarity.

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  • \$\begingroup\$ That's a great picture, I could not "visualize" the field doing that. However, as you pointed out, the field will be weaker as you get away from the conductor, so wouldn't the left-side of the ring catch stronger lines than the right-side? !picture \$\endgroup\$ – Zmaster Jun 13 at 15:58
  • \$\begingroup\$ There will always be a stronger mag field on the left side but the net effect with distance is very little. I wish I could summon up the math but it’s beyond my capabilities. \$\endgroup\$ – Andy aka Jun 13 at 20:03

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