zero-magnethttp://www.oed.com/dictionary/zero-magnet_n%3Ftab%3Dmeaning_and_usezero magnet, n.
Revised 2018

zero magnetnoun

Now rare.
  1. 1890–
    A magnet used to reset the zero of an instrument or device; cf. zero n. A.4a.
    1. 1890
      Deflecting-magnet,..also called zero magnet, directing-magnet, and deflector.
      Century Dictionary at Magnet
    2. 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
    3. 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

Originally published as part of the entry for zero, n. & adj.

zero, n. & adj. was revised in June 2018.