A magnet used to reset the zero of an instrument or device; cf. zeron. A.4a.
1890
Deflecting-magnet,..also called zero magnet, directing-magnet, and deflector.
Century Dictionary at Magnet
1894
I use a small weak ‘zero magnet’, with its magnetic axis perpendicular to the plane of the mirror, for the purpose of bringing the zero of the scale to the cross wire of the telescope.
Philosophical Transactions 1893 (Royal Society) A. vol. 184 596
1969
In a variation of the instrument an additional zero magnet causes the pointer to return to the low end of the scale.
W. W. Bischoff in J. G. Giles, Vehicle Equipm. v. 222