caduceus coil building and testing

[bistander]

I don't know where to post this?
My compass has reversed polarity.
How can this be possible?

Hi ragged,

The Earth's poles have finally flipped.

Just kidding.

Your compass needle, or pointer, has been remagnetized in the opposite direction. The pointer is a ferromagnetic material (iron, steel, magnetite) which has retentivity, or property of residual magnetism, meaning once subjected to a magnetic field its domains will align with that direction of external field and to some degree remain in that aligned position even after the external field is removed, thus becoming a permanent magnet in itself. Well, permanent until demagnetized. Demagnetization can occur due to several conditions in including opposite external field of sufficient strength, alternating external field, age, mechanical shock, Curie temperature, etc. Once demagnetized, it can be easily magnetized in the opposite direction. I'll attach a typical hysteresis curve from this article: https://experimentationlab.berkeley.edu/hysteresis

The most likely scenario is that your compass was forced quickly into an external field, like near a strong magnet or wire/coil with high DC, having the pointer in the opposite position and that external field reorienting the magnetic domains before the inertia of the pointer (and dampening friction) allowed the pointer to swing 180°. I've seen in happen often. You can correct the pointer magnetic direction with this method also.

Hope that helps.
bi