Inequality Of Temperature
In a various type of circumstances, when system S3 is in thermal equilibrium with S1, though whenever it is brought into contact with the other system S2, noticeable changes occur in S3, (in the absence of any work communication), then the temperatures of S1 and S2 are thought to be unequal.
In order to ease the recognition of temperature inequality, the system S3 is so chosen that changes in its physical characteristics can be simply noticed. For illustration, in the generally used mercury-in-glass thermometer, the thermometric property is the volumetric expansion of mercury associative to that of glass. In the above experiment, a difference in the length of the mercury column, if S3 contacts S1 and S2 in progression, means that S1 and S2 have unequal temperatures. On the other hand, when there had been no change in the length of the mercury column, this would mean that S1 and S2 have equivalent temperatures.