Electro-magnetism describes the relationship between electricity and magnetism — electricity produces magnetism. Scientists hoped to find that the reverse was also true and many tried to demonstrate it. Faraday persisted longer than most, and finally succeeded on 29th August, 1831. His set up was equivalent to have a continuous ring in the middle made of solid soft iron. It’s wrapped on opposite sides in two sets of unconnected copper wires. Let’s call the left hand side coil A and the right hand side coil B. The wires don’t touch each other or the ring at any point. They are wrapped around the ring in layers of cotton wadding for insulation. The coils have 2mm spacing between each winding. Each end of coil A is connected to a battery, which will provide the current. Each end of coil B is connected to a galvanometer, an instrument that detects current.

When the battery is connected up, the needle of the galvanometer leaps into action, registering current in coil B. However, the effect quickly fades and the needle soon detects no current, even though the battery is still connected. If the battery is switched off and on again repeatedly, the effect can be reproduced over and over again, as rapidly as you like. When the battery is connected up, electrons flow along the copper wire of coil A, round the windings round the ring. The effect of this is to induce magnetism in the ring. A magnetic field, or vibration field, of excited electrons is created, producing an electrical current in coil B, which is inside the magnetic field. This is one of Farady’s great discoveries — electro-magnetic induction.

Repeatedly switching the power on and off generates what we call alternating current (AC), since the current swaps back and forth between the two coils. This principle is the basis for much of our modern public electricity supply. In setting up this experiment, Faraday invented the transformer — his apparatus was a primitve version of the transformers we use today, in everything from electricity substations to mobile phone chargers.

 

Reference:

http://www.historyorb.com/day/august/29

http://www.engineering-timelines.com/how/electricity/transformer.asp