Before knowing about iron box, heating effect of current has to be known. When current passes through the wire or coil, it produces heat in the wire. Here electrical energy is converted to heat energy. Metallic conductor can be any wire or coil.

If you explore metallic conductor, we will come to know the information about free electrons.A metallic conductor has a large number of free electrons in it. When potential difference or voltage is applied between ends of metallic conductor, free electrons move from region of low potential to high potential.

While moving, electrons collide with atoms and due to these collision, energy is transferred to atoms . Finally, atoms commence to vibrate and as a result of that, heat is produced.

If the number of electrons flowing per second are high, then rate of collisions will be high. Consequently, more heat will be produced. Thus, heating effect of current is known. This effect is applied in iron box.

Ironbox contain coils of high resistance wire made of nichrome alloy. When this appliance is connected to power supply by insulated copper wires, then a large amount of heat is produced in the heating coils because they have high resistance and Coils are not affected for high temperatures. A negligible heat is produced in the connecting wires because the wires are made to have low resistance.