there are two types of thermodynamic instruments, the meter and the reservoir. a thermodynamic meter is any device which measures any parameter of a thermodynamic system. in some cases, the thermodynamic parameter is actually defined in terms of an idealized measuring instrument. for example, the zeroth law states that if two bodies are in thermodynamic equilibrium with a third body, they are also in thermodynamic equilibrium with each other. this principle, as noted by james maxwell in 1872, asserts that it is possible to measure temperature. an idealized thermometer is a sample of an ideal gas at constant pressure.