Ans. The condition in which two substances in physical contact change or transfer no heat, is called Thermal equilibrium. Here two substances are stated to be at the same temperature in thermal equilibrium.
When the gas from a plucker tube passes through an electric discharge, there should not be any doubt whether the partition of energy is different from thermal equilibrium.
The staggered heating cycle was invented to deal with the change in thermal equilibrium. In contrast, the thermal equilibrium between vapour and liquid was made by Lord Kevin and afterwards Thomson.