= 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 $\mathcal E=-N\,d\Phi/dt$.
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.
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