The Choke
The current in a dc circuit is controlled with a rheostat or variable ressitor, but this is very inefficient. A lot of energy is wasted as heat in the rheostat. The current in an ac circuit can also be controlled with a rheostat but it is more efficient to use a choke. A choke consists of a low resistance copper coil wound on a laminated iron core.
When ac passes through the coil it sets up a changing magnetic field in the iron core. The changing magnetic field in turn sets up a changing EMF in the coil, which opposes the supply EMF. This is Lenz's Law:
The direction of the induced EMF is such as to oppose the change calling it
This phenomenon of an instrument inducing an EMF in intself is called self induction.
The counter EMF impedes the current without leading to the waste of energy that occurs when a current flows against a resistance. Still the back EMF leads to the reduction in the current in a similar way to resistance. The property of an instrument to impede current because of its own self inductance in this way is called inductive reactance.
