Known as Palmieri's Apparatus (Luigi Palmieri, Italian physicist) or Delzenne's Ring (Charles Edouard Joseph Delzenne, French). It is an apparatus for showing the currents induced by the terrestrial magnetic field, with commutator and slip rings for taking off direct and alternating current. It is formed by a circular wooden frame of a 261mm diameter, with hundred turns of insulated copper wire. The two ends of the coil are connected with a ring collector, that works as a rectifier of the alternate current in its medium part. The frame is applied to the rotational apparatus. If we lay the two brushes on the extreme edges of the collector, we obtain an alternate current; if we lay them on the medium part, we obtain an induced rectified current.
If a ring like that is mounted in a rotable frame we can calculate the size and direction of the magnetic field of Earth. The experiment is done with the coil horizontal (to find the vertical component of the earth's field) and vertical (to find the horizontal component). The direction and net field can then be readily calculated.
Perhaps it was made by Max Kohl (No 52064)