Dc supply to transformer

Transformer work only with AC voltage. For DC transformer act just like a short wire (a very small resistance ).

 

Actually this sounds like a disguised homework question. If you are messing with transformers in lab you should already know the answer to the question.

 

Because transformers is essentially just two or more coils of wire wrapped around the core. And wires has "low" resistance.

 

You have a careful smart friend

 

Actually, if your friend had been able to measure the way the current varied around the time of switch on, he would have found that the current value followed a curve, because the Iron core of the transformer behaves like an electromagnet. The magnetic field energy takes time to build up and it produces a 'back emf' which delays the DC current increase (This would be fairly quick with a mains transformer - only a few tens of milliseconds before the fuse started to blow.)