Step 1: Take two coils placed close to each other. Mutual induction is the property by which each coil opposes a change of current in the other by inducing an emf in it; the coils are then said to be magnetically coupled.
Step 2: Define the coefficient of mutual inductance \(M\) through the induced emf. If the primary current changes at a rate \(dI_1/dt\), the secondary emf is \(\varepsilon_2 = -M\,\dfrac{dI_1}{dt}\).
Step 3: Rearranging, \(M = \dfrac{|\varepsilon_2|}{|dI_1/dt|}\). So \(M\) is numerically the emf (in volt) induced in the secondary when the primary current changes at the rate of \(1\) ampere per second.
Step 4: The coefficient is symmetric, \(M_{12} = M_{21} = M\), so one number describes the coupling in either direction.
Step 5: From \(M = \dfrac{\varepsilon_2}{dI_1/dt}\) the unit is \(\dfrac{\text{volt}}{\text{ampere/second}} = \text{volt·second/ampere} = \text{henry}\).
\[\boxed{M \text{ is measured in henry (H)}}\]