Hold the magnet still inside the coil and the meter reads zero, however strong the field is. Only the change in flux drives a current. Drag the magnet through the coil and watch the needle swing — then swing back as it leaves.
whatA bar magnet and a coil. Flux is integrated numerically across the coil disk every frame, then differentiated in time — the meter reads that derivative, not the field.
lawε = −N dΦ/dt. The minus sign is Lenz's law: the induced current opposes the change that made it.
tryPark the magnet in the middle of the coil, then pull it out fast.
noticeHeld still, the EMF is zero even though the field is strongest. Moving through, the trace peaks on entry, crosses zero at the centre, and reverses on the way out.
in lifeEvery phone charging pad, metal detector and guitar pickup works on this and nothing else.
found byFaraday, 1831, working with iron rings and coils. Lenz added the minus sign in 1834.