• Place two coils near each other (or one inside the other) the primary, joined to a battery, switch and rheostat, and the secondary, joined to a sensitive galvanometer with a central zero.
• A pointer deflection occurs in the secondary when:

• the primary is brought close to (or pushed into) the secondary; the deflection is reversed when it is withdrawn;
• the primary is inside the secondary and the current in the primary is increased (moving the rheostat); the deflection reverses when it is decreased;
• the primary circuit is closed (current grows from zero); the deflection reverses when it is opened.
• No deflection when the primary current is steady (the flux is not changing), and none when there is no current in the primary.

• Interpretation when the primary current rises (or the primary approaches, or is switched on), the flux linked with the secondary rises and the induced current in the secondary is in the direction opposite to the primary current (its field opposes the increase).
• When the primary current falls (or the primary is withdrawn, or is switched off), the induced current is in the same direction as the primary current (its field tries to maintain the flux).
• This agrees with Lenz’s rule, and the experiment is also used to verify it.
• Mutual induction is the production of an emf in one coil by a change of current in a neighbouring coil.

1. The flux linked with the secondary is proportional to the primary current
2. so the induced emf is proportional to the rate of change of the primary current:

emfsecondary = − M ΔIprimary / Δt

• M is the coefficient of mutual induction (mutual inductance).
• Its unit is the henry (H) = V·s/A.
• A pair of coils has a mutual inductance of 1 H if an emf of 1 V is induced in the secondary when the current in the primary changes at 1 A/s.
• The henry is the unit used to measure inductance in general.

• M depends on the presence and type of an iron core inside the coils; the volume (size) of the coils and the number of their turns; the distance between them (and their relative orientation).
• The transformer is a clear example of mutual induction, and the induction coil is another application.
• Induction coil (brief):

• a device with a primary of few thick turns around a soft-iron core and a secondary with very many thin turns.
• An interrupter repeatedly breaks the DC in the primary; each break induces a very large emf in the secondary, giving a high voltage from a low-voltage battery (spark coils, ignition).