Transformer works on the principle of mutual induction which is a special type of electromagnetic induction wherein a current change in primary coil induces current in secondary coil. A D.C. source provides a constant current which does not produce any change in magnetic flux. Thus if D.C. is used in the primary coil there will be no change in the magnetic flux linked with the secondary coil. As a consequence no emf will be induced in the secondary coil.
