 Pickup coil

A pickup coil converts changing magnetic flux into an electromotive force through Faraday's law of induction. For $N$ identical turns linking flux $\Phi (t)$ per turn, the ideal open-circuit signal is $\mathcal{E}=-Nd\Phi /dt$.

Move a magnet toward the coil, stop it and withdraw it: approach and withdrawal give opposite signal signs, while a stationary magnet gives no ideal induction signal despite its nonzero magnetic field. This makes the coil a change detector rather than a direct static-field meter. Coil area, orientation, winding resistance and readout loading determine the actual voltage and noise; integrating a calibrated signal can infer a change of flux.