The heating effect of electric current is defined by the Joules Law of heating.It states that the heat H produced by a resistor of resistance R due to current I flowing through for a time t as :
H = i² Rt
In a conductor when an electric field is applied across its ends , the free electrons available in it start drifting along the electric field.These electrons collide with the atoms which have lost the electrons.As a result of these collisions , some energy of the electrons is transferred , to the atoms which vibrate violenty as they gain energy.Thus , heat is developed in the conductor.
Greater the current , greater will be the rate of collision and so greater will be the  heat produced.Electric Iron is one such device and works on this principle.

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