Long a standard tool in the electronics shop, the versatile oscilloscope uses an electron beam to trace the voltage as a function of time. While that is the standard application, a second voltage can be applied to the horizontal deflection plates instead of the timing ramp voltage to produce an xy plot.
Electrons released by thermionic
emission from the hot filament in the
electron gun are accelerated by a high
voltage and focused to produce an energetic
electron beam. The phosphor glows at the point
of impact of the electron beam, and that glow is persistent enough for you to be able to see the entire trace of the electron beam across the screen.