Rhumkorff Induction Coil

It is a device by which a low voltage direct current can be converted into a high voltage pulsating current. It works on the principle of mutual induction between a pair of coils.

Construction. It consists of the following parts.
(1)    Primary coil. It consists of a few turns of thick insulated copper wire would on a hollow wooden cylinder. The primary coil is connected in series with a battery and a “make and break arrangement.”

(2)    Secondary coil. A secondary coil consisting of a large number of turns of thin insulated copper wire is tightly wound on the primary coil. The resistance of the secondary coil is very high. The ends of this coil are connected to a spark gap G₁G₂ of adjustable width.

(3)    Iron core. It consists of bundle of soft iron wires insulated from each other and is placed inside the hollow wooden cylinder, parallel to its axis. The iron core increases the magnetic flux produced by the primary.

(4)    Automatic make and break arrangement (Interrupter). It consists of a soft iron hammer H supported by a spring. At the back of H, there is a platinum tip P1.There is an adjustable screw B which also has a fine platinum tip P2. The screw can be adjusted until P1 and P2. The screw can be adjusted until P1 and P2 are in contact gets closed.

(5)    Capacitor. A parallel plate capacitor C is connected between P1 and P2



Action. When the key K is pressed, a current flows in the primary and magnetises the iron core. The iron core attracts the head H. The contact between P1 and P2 is broken. Now the current in the primary falls to zero and the iron core is demagnetized. Hence the head H flies back to its initial position and the contact between P1 and P2 is made once again. Once again current passes through the primary. This process of make and break of the primary current is repeated with a large frequency depending upon the stiffness of the spring.

Generation of High P.D. At make, a high magnetic flux is established in the soft iron core. This change of flux links with the secondary also and so an emf is induced in the secondary. This is called induced emf at make.

When the primary current is broken , the primary current decays rapidly resulting in the decrease of magnetic flux linked with the secondary. This induces a very high emf in the secondary but in the reverse direction, to that developed at the time of make. This is called the induced e.m.f. in the secondary at break.

Let R be the total resistance in the primary circuit and L the inductance of the primary coil. The self-inductance of the primary opposes the growth as well as the decay of current. The resistance of the primary circuit at break is much larger than that at make. Therefore, the time constant (L/R) of the circuit at break is shorter than that at make. This is turn results in the induced e.m.f. at break being much lower than that at make.



Action of the capacitor. The primary resistance at break is almost entirely due to the air gap between the platinum points in the interrupter. The entire induced emf due to self-inductance of primary, therefore, appears across this air gap. This causes intense sparking, if capacitor C is absent. Hence the platinum points will be damaged.

To prevent this defect, a capacitor C of large capacitance is connected is parallel with the spark gap of the interruptor. The e.m.f. produce in the primary at break due to its self-induction is used up in charging the capacitor and therefore the sparking across the air gap is considerably reduced. The capacitor then discharges through the primary giving rise to a current in a direction opposite to that of the battery current. Actually the capacitance and inductance of the primary circuit may be so large compared with the resistance that the discharge is oscillatory. But due to large damping, the oscillations are dead-beat. We can, therefore, consider the capacitor to have discharged completely in between two consecutive makes.