The entropy of a substance can be measured, although in an indirect way. The measurement, known as entropymetry, is done on a closed system with constant number of particles N and constant volume V , and it uses the definition of temperature in terms of entropy, while limiting energy exchange to heat d U → → d Q : T := ( ∂ ∂ U ∂ ∂ S ) V , N ⇒ ⇒ ⋯ ⋯ ⇒ ⇒ d S = d Q T The resulting relation describes how entropy changes d S when a small amount of energy d Q is introduced into the system at a certain temperature T .