3 is
as follows: When the tapping key K is pressed, the current in
coil C2 (and the resulting magnetic field) rises from zero to a
maximum value in a short time Consequently, the magnetic
flux through the neighbouring coil C1 also increases It is the change in
magnetic flux through coil C1 that produces an induced emf in coil C1 When the key is held pressed, current in coil C2 is constant