The Zeroth Law justifies the concept of temperature and the use of a thermometer,
a device for measuring the temperature. Thus, suppose that B is a glass capillary
containing a liquid, such as mercury, that expands significantly as the temperature
increases. Then, when A is in contact with B, the mercury column in the latter has
a certain length. According to the Zeroth Law, if the mercury column in B has the
same length when it is placed in thermal contact with another object C, then we can
predict that no change of state of A and C will occur when they are in thermal contact.
Moreover, we can use the length of the mercury column as a measure of the temperatures
of A and C.