A cathode ray tube (CRT) creates an electron beam that is seen as a dot.

It can be moved with a power supply, which creates a potential difference to alter the path of the beam in a direction that depends on its polarity. (this can be seen here: https://www.youtube.com/watch?v=TYnZC3Ay3P0&feature=youtu.be)

It can also be moved with a magnet, which also alters the path of the beam depending on its polarity. (this can be seen here: ...