If such a magnet is spun about its axis of symmetry, then each constituent microscopic current loop translates in its plane. Each tiny loop accordingly has an electric dipole moment that points radially toward or away from the spin axis. The net electric field of these myriad microscopic current loops is conservative and has a radial component above and below the magnet.

It is of historic (pre-relativistic) note that, since dB/dt=0 in this case (both when the magnet is at rest and when it is spun), the existence of the electric field in the spinning case was either (a) unsuspected, or (b) if suspected, was of unknown origin. That is, the electric polarization of each microscopic current loop, when the parent magnet is spun, is strictly a relativistic effect.

Fig. 4.2_1 depicts a conducting probe above a disc-shaped magnet. The probe is attached to a dielectric shaft, and it slides on a conducting ring. The magnet can be independently spun on another shaft.

Let us first spin the probe above the resting magnet. Viewed from above, we shall spin the probe CCW. Conduction electrons in the probe are of course driven toward the probe shaft. An electroscope indicates that the ring has acquired a positive charge.

We next keep the probe at rest and spin the magnet CCW. Each microscopic current loop in the magnet is electrically polarized, and the net electric field has radially inward pointing components. An electroscope should reveal a negatively charged ring.

The electric forces on probe conduction electrons oppose the magnetic forces. To the extent they cancel completely, an electroscope should indicate no charge on the ring.

Fig. 4.3_1 depicts the assembly of Fig. 4.2_1, but with a closing wire completing the shaft/probe/ring/closing wire circuit. (The shaft is now conducting. The closing wire slips on, but makes electrical contact with, the shaft.)

If the magnet is held at rest and the probe is spun, then there will of course be a current.

If the probe is held at rest and the magnet is spun, then no current should flow. This is because the spinning magnet’s electric field is conservative, and the emf around the circuit is zero.

If the probe and magnet are spun in tandem, then a current can be expected. The magnetic and electric forces on probe conduction electrons hypothetically sum to zero, and the emf in this part of the circuit is zero. This leaves the emf in the closing wire/shaft part of the circuit unopposed and a current should flow CCW around the circuit.