An oscilloscope is a device for displaying signals that are too fast to be seen by the human eye. Typically the signal consists of a voltage level that changes quickly moment by moment (over millisecond to nanosecond timescales). What is seen on the screen of the scope is a waveform whose value is graphed along the vertical axis as a function of the horizontal axis representing time. An electron beam, aimed at a phosphorescent screen, is swept horizontally providing a light-trace on the screen while, coincidentally, the instantaneous voltage of the input signal is used to deflect the electron beam up or down, creating the visible trace.