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An oscilloscope is a device that shows progressive advances of a time-dependent waveform. It is very useful in both science and technology. The wave does not need to be completely periodic but it generally is. The display has two modes. It can be free-running at a chosen update frequency. Or it can be triggered by a special feature of the waveform. The triggering is typically an input voltage value that can be set by the user.
The basis of an oscilloscope is often a Cathode Ray Tube More modern versions use digital screens where the screen is defined by a n by m grid of light emitting diodes. That n by m grid is usually backed up by memory elements.