13 Jul 2026

Ratio of Specific Heats by Clement and Desormes' Method

practical ug-v thermal-physics specific-heats gas

Experimental arrangement

Clement--Desormes arrangement: the gas vessel is brought to the initial temperature, briefly opened, and allowed to return to thermal equilibrium.

Aim

To determine the ratio of specific heats $\gamma=C_p/C_v$ of a gas by Clement and Desormes’ method.

Apparatus

Large glass vessel, pressure gauge, stopcock, pump, and connecting tubes.

Theory

The gas is first compressed slowly to pressure $P_1$. It is then allowed to expand rapidly to atmospheric pressure and finally warmed at constant volume to pressure $P_2$. If $h_1$ is the initial excess pressure and $h_2$ is the final excess pressure,

\[\gamma=\frac{h_1}{h_1-h_2}.\]

Observations

Trial	Initial excess pressure $h_1$ (cm Hg)	Final excess pressure $h_2$ (cm Hg)	$\gamma$
1	18.2	5.4	1.422
2	18.0	5.3	1.418
3	18.1	5.4	1.425

Result

\[\boxed{\gamma=1.42}.\]

Precautions

1. The expansion should be rapid compared with heat exchange.
2. The final reading should be taken only after thermal equilibrium.
3. Close the stopcock without leakage.
4. Avoid parallax in the manometer reading.

Viva Questions

1. Why is the expansion rapid? It is treated as adiabatic during the expansion.
2. Why is the final reading delayed? The gas must return to the temperature of the surroundings.
3. What is the value of $\gamma$ for a monatomic ideal gas? $5/3$.

Maxima Code

Download the Maxima calculation file.