Explanation for those who don’t want to watch the video:
Spoiler for those that still plan to
The magnetic field generated by the batteries was so insanely strong, many non-aligned compass needles were, instead of being turned around, permanently re-polarized to conform to the now present magnetic field.
tl;dr: Exposing compasses to fusion reactor level strong magnetic fields will break (some of) them.
Explanation for those who don’t want to watch the video:
Spoiler for those that still plan to
The magnetic field generated by the batteries was so insanely strong, many non-aligned compass needles were, instead of being turned around, permanently re-polarized to conform to the now present magnetic field.
tl;dr: Exposing compasses to fusion reactor level strong magnetic fields will break (some of) them.