Three linked loops make up the transformer
A coil of current-carrying wire produces a steady magnetic field, just like a bar magnet. Link this magnetic field to a second coil by placing that second coil near the first. You'll get a good link when the strength of the magnetic field inside the second coil is as big as possible. Use what you know about the shape of the magnetic field produced to guide the placement in order to maximise the linkage. The linkage between the coils can be improved further by using a soft iron loop, because this provides a preferred route for the magnetic field, just as copper wire does for the electrical current.
The magnetic effect propagates round this loop, just as electrical effects flow round copper loops, but nothing magnetic flows. So now you have two interlinked loops: the electrical loop and the magnetic loop. But nothing is happening in the second coil – a steady current in the first electrical loop produces a steady magnetic field in the iron loop, which doesn't induce any activity in the second coil.
It took the persistence and genius of Faraday, working with his hand-made apparatus, to show that only a changing magnetic field induced electrical activity in the third loop.
What each loop does
Changing the current in the first coil produces a changing magnetic field. An alternating current produces an alternating field. You can drive an alternating current through the first coil by applying an alternating potential difference across this input coil. The electrical current in the input coil sets the value of the magnetic field produced by that input coil.
The simplest kind of change to explore is where the current changes steadily with time, say increasing at 0.1 ampere second -1. This steadily increasing electrical flow round the input electrical loop will produce a steadily increasing magnetic effect in the magnetic loop.
Now we turn to the second electrical loop – the output coil – to explore the experimental findings of Faraday.
Steady changes induce constant outputs
A steadily increasing magnetic effect in the second coil induces a constant potential difference across the second coil. So the second coil acts as if it is a battery of constant potential difference. As a result, the second coil, if connected into an electrical loop with a resistor, has an electrical current driven around it (I = VR). So a steadily increasing electrical flow in the input coil produces a steadily increasing magnetic effect, which induces a constant potential difference in the linked output coil.
A constant electrical flow in the input coil produces a constant magnetic effect, which induces no potential difference in any linked output coils.
However, a steadily decreasing electrical flow in the input coil, driven by a steadily decreasing potential difference across that coil, produces a steadily decreasing magnetic effect. Any linked output coils now have a constant negative potential difference induced across them, resulting in a reversed constant electrical flow in completed electrical loops.
Coils linked to a changing magnetic loop can be induced to act like electrical batteries.
If the magnetic effect is increasing then the polarity of the
battery is in one sense (one end of the coil acts as a positive terminal, the other as a negative terminal); if the magnetic effect is decreasing, then the terminals of the
battery have to be switched.
Now add a resistor, or any other load, to the output coil to make a complete loop.