Pulse width modulation (PWM) is a more efficient technique than linear control for driving solenoids. PWM works by turning the signal on for part of its period and off for the rest, varying the duty cycle. If the duty cycle is shorter than the solenoid's rise time, the current will be discontinuous and not reach its maximum; if longer, the current will be continuous but have ripple. At high frequencies, the current becomes essentially constant by adjusting the duty cycle. Dither can also be added to counter stiction and hysteresis in hydraulic valves, producing small vibrations in the solenoid current. High frequency PWM allows independent control of current level and dither properties.