An oscilloscope or scope is an electronic measuring instrument that creates a visible two-dimensional graph of one or more electrical potential differences. The horizontal axis of the display normally represents time, making the instrument useful for displaying periodic signals. The vertical axis usually shows voltage. The display is caused by a "spot" that periodically "sweeps" the screen from left to right.
Analogue storage oscilloscope
An extra feature available on some analogue scopes is called 'storage'. This feature allows the trace pattern that normally decays in a fraction of a second to remain on the screen for several minutes or longer. An electrical circuit can then be deliberately activated to store and erase the trace on the screen.