I placed two iron bars together inside a big solenoid. I had previously shown that they were not magnetic. The electric current was turned off. Then I flicked the switch and the iron bars repelled each other. What was going on?
Magnetic materials contain magnetised areas called domains. These are like tiny compasses inside the material. We say that the metal is unmagnetised when the domains are pointing in random directions so that they cancel each other out. But when the switch is flicked, the solenoid coil becomes an electromagnet and lines up the domains. Lined up domains act like an ordinary bar magnet. The two iron bars are now like two bar magnets lying side by side with the same poles at each end. They repel.
