Faraday, Lenz and mutual induction

Put two coils close together. The primary is joined to a cell through a switch (and perhaps a rheostat); the secondary is joined to a zero-centre galvanometer and has no cell. The current in the primary makes a field, and part of its flux passes through the secondary. If the primary current is steady, the flux is steady and the secondary shows nothing. The galvanometer deflects only while the primary current changes: when the switch is closed or opened, when the rheostat slider is moved, or when one coil is moved towards or away from the other. The induced emf is emf = −M ΔI ÷ Δt, where ΔI ÷ Δt tells how fast the primary current changes and M is the mutual inductance in henry (H). M is larger with an iron core, with more turns and bigger coils, and when the coils are close together. Lenz’s rule gives the direction: when the primary current rises, the induced current in the secondary flows against the direction of the primary current; when it falls, it flows in the same direction as the primary current.