In general, the time interval required for a system exposed to some discontinuous change of environment to undergo the fraction (1 − e⁻¹), or about 63%, of the total change of state that it would exhibit after an infinitely long time. For example, a thermometer initially at equilibrium in a bath at temperature T₁ will exhibit an exponential change of temperature with time after being suddenly plunged into a bath at temperature T₂, theoretically assuming the new temperature T₂ only after an infinitely long time. The finite time interval required for the thermometer to undergo a change of amount (T₁ − T₂)(1 − e⁻¹) is called the thermal relaxation time of the thermometer. Occasionally, the fraction 9/10 is used in place of (1 − e⁻¹), so contexts must always be checked to be certain of the definition employed in a given case. The definition may also change for an underdamped device. The change of state of such a device may oscillate several times while approaching its final value.