(a) Whenever there is a change in the magnetic flux linked with a circuit, an electromotive force is induced, (1\(\frac{1}{2}\)) the strength of which is proportional to the rate of change of the flux linked with the circuit (1\(\frac{1}{2}\)).
E \(\alpha\) \(\frac{d \theta}{dt} or E = \frac{Nd \theta}{dt}\)

Where E = induced e.m.f.
\(\theta\) = magnetic flux.

(b)

Workability: All except key and capacitor. When the platinum contacts touch ar,d the key k is closed, current passes through the primary coil and the core is magnetised. The soft iron armature is then attracted so the, the platinum contacts separate and the circuit is broken. When this happens, the core loses its magnetism and the armature is pulled back by the spring so that contact is re-established. As the contact is repeatedly made and broken in the primary circuit, changes of magnetic flux take place and induces an e.m.f. in the secondary coil. Because of the much larger number of turns in the secondary compared with the primary, a very high e.m.f is induced in the secondary coil which is sufficient to cause a spark to jump across the spark gap.
(c) The core of the coil is made of bundle of soft-iron wires OR that are of the coil is laminated. Or use of particle of iron dust. Or lamination of iron sheet or iron core not metallic sheet

(d) – To control sparking at the platinum contacts (which otherwise wears off the contact points)
-- It promotes a rapid decay of the magnetic flux at the break of the primary circuit and this helps to increase the secondary e.m.f.

(e) It is used in the
– coil
– ignition system of motor cars
– inveitigation of high voltages
– study of electric discharge through gases
– operation of x-ray tubes
– merse radio transmitter.