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 R1, R2, R3 etc. The corresponding quantities of heat gained H1, H2, H3 etc. are calculated. It is found that,
H1/ R1 = H2/ R2 = H3/ R3
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 t1, t2, t3 etc. The corresponding quantities of heat gained H1, H2, H3 etc. are calculated.
It is found that
H1/ t1 = H2/ t2 = H3/ t3
H/t = constant
i.e H α t. Hence, law of time is verified.