If the equilibrium mixture of a chemical reaction is disturbed by a sudden change, especially of some external parameter (such as temperature, pressure or electrical field strength), the system will readjust itself to a new position of the chemical equilibrium or return to the original position, if the perturbation is temporary. The readjustment is known as chemical relaxation. In many cases, and in particular when the displacement from equilibrium is slight, the progress of the system towards equilibrium can be expressed as a first-order law: where and are the equilibrium concentrations of one of the chemical species involved in the reaction before and after the change in the external parameter, and is its concentration at time . The time parameter , named relaxation time, is related to the rate constants of the chemical reaction involved. Measurements of the relaxation times by relaxation methods [involving a temperature jump (T-jump), pressure jump, electric field jump or a periodic disturbance of an external parameter, as in ultrasonic techniques] are commonly used to follow the kinetics of very fast reactions.