Joule’s law states that the amount of heat produced in a current conducting wire, is proportional to the square of the amount of current that is flowing through the wire, the electrical resistance of the wire and the time of current flowing. $$E∝I^2Rt$$ $$E=\frac{1}{J}I^2Rt$$ J = Joule’s Mechanical Equivalent of Energy = 4.2 J / cal This can be calculated by using the energy lost from the circuit to heat up another body whose specific heat capacity is known.
Here, water is chosen for heating since we know that it remains in the same phase at our required temperature range and the difference between its melting point and boiling point is large enough for experimentation.
There are broadly three ways in which heat is transferred- conduction, convection and radiation.

1. Conduction is heat transfer via vibration of particles in their mean position. First the particles in contact with the source of energy start vibrating. Vibration of these particles causes its neighboring particles to vibrate. And energy flows throughout the body in this way.
2. Convection is the movement caused within a fluid by the tendency of hotter and therefore less dense material to rise, and colder, denser material to sink under the influence of gravity, which consequently results in transfer of heat.
3. Radiation is heating due to emission of electromagnetic waves Conduction and convection require matter for transferring heat. The isolation chamber keeps the system of away from good conductors of heat. However, since radiation does not require matter for heat loss, the isolation chamber cannot stop heat loss via radiation. This heat loss has to has to be incorporated in the conservation of energy equation. For finding energy lost due to radiation. The system is allowed to cool down in the same isolated chamber for half the time it took for heating and the temperature is noted.