The charge time-constant of a peak rectifier is commonly specified in terms of the rate of rise of voltage with time when a sinusoidal signal is suddenly applied. An expression for rise time is derived, particularly for the case in which the time of one cycle of a.c. is small compared with the charge time-constant of the rectified system; the time then taken for the rectified voltage to reach any given percentage of its steady-state value is shown graphically. In particular, an a.c. signal takes 2.5 times as long to reach 80% of its steady-state value as d.c. step of the same amplitude.