ANDREW LAMBERT PHOTOGRAPHY / SCIENCE PHOTO LIBRARY ANDREW LAMBERT PHOTOGRAPHY / SCIENCE PHOTO LIBRARY
AC and DC power supplies displayed on the screen of an oscilloscope (centre). Direct current (DC, the straight line) is produced by the equipment at left, while alternating current (AC, waveform) is produced by the equipment at right. Both currents are powering lamps at lower centre. The grey box (left) is a smoothing unit that is needed to form smooth DC from AC. Many electrical devices need to have the mains AC power supply altered in this way to run safely. The DC supply is seen to be about 0.7 times the peak AC value. This is known as the RMS (root mean square) value.
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