Single-phase mercury-arc rectifiers were rarely used because the current dropped and the arc could be extinguished when the AC voltage changed polarity. The direct current produced by a single-phase rectifier thus contained a varying component (ripple) at twice the power supply frequency, which was undesirable in many applications for DC. The solution was to use two-, three-, or even six-phase AC power supplies so that the rectified current would maintain a more constant voltage level.