Joule’s law is verified using Joule’s calorimeter. It consists of a resistance coil R enclosed inside a copper calorimeter (Figure).

Fig: Joule’s calorimeter

The ends of the coil are connected to two terminals, fixed to the lid of the calorimeter. A stirrer and a thermometer T are inserted through two holes in the lid. Two thirds of the volume of the calorimeter is filled with water. The calorimeter is enclosed in a wooden box to minimize loss of heat.

A battery (Bt), a key (K), a rheostat (Rh) and an ammeter (A) are connected in series with the calorimeter. A voltmeter (V) is connected across the ends of the coil R.

**Verification of Joule’s Law**

**Law of resistance:**

The same amount of current I is passed for the same time t through different coils of resistances R_{1}, R_{2}, R_{3} etc. The corresponding quantities of heat gained H_{1}, H_{2}, H_{3} etc. are calculated. It is found that,

H_{1}/ R_{1} = H_{2}/ R_{2} = H_{3}/ R_{3}

H/R = Constant

i.e H α R. Hence, law of resistance is verified.

**Law of time:**

The same amount of current I is passed through the same resistance R for different intervals of time t_{1}, t_{2}, t_{3} etc. The corresponding quantities of heat gained H_{1}, H_{2}, H_{3} etc. are calculated.

It is found that

H_{1}/ t_{1} = H_{2}/ t_{2} = H_{3}/ t_{3}

H/t = constant

i.e H α t. Hence, law of time is verified.