The device used to provide electricity at the proper voltage for household purpose is generator. Principle of working of generator:

Generator works on Faraday's Law of electromagnetic induction. This law states that when a conductor moves in a magnetic field it cuts magnetic lines of force, which induces an electromagnetic force (EMF) in the conductor. The magnitude of this induced EMF depends upon the rate of change of flux (magnetic line force) linkage with the conductor. This EMF causes a current to flow when the conductor circuit is closed.

Quite often there remain air gap between the magnetic poles and the armature. Leakage flux occurs here causing the loss of energy and hence loss in efficiency of the generator.
To reduce the flux leakage:
(i) Air gap to be minimised
(ii) Curvature of magnetic poles is made same to the curvature of the rotating armature.