What is a cathode ray oscilloscope?
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At its core is a cathode ray tube – a device which fires electrons towards a screen which lights up when and where the electrons hit. On its own this is a bit boring because you just get a bright green dot in the centre of the screen.
The clever thing is that you can steer the beam of electrons with magnetic fields, moving the dot around or producing a line (because the screen stays glowing for a moment). The oscilloscope allows you to provide inputs to generate these magnetic fields and so visualise those inputs as lines.
The oscilloscope will also have various hardware to manipulate the effect. For example you can amplify the input signal and so change the scale of the display. Or very commonly you only want one input, so you set the other to simply ramp repeatedly in a fixed period of time. This causes the beam to scan across the screen in a horizontal line. Your input is then visible as vertical changes to this line.
If your input is a repeating signal (it often is, hence “oscillo-” from oscillating) then you adjust the time period of the horizontal scanning until it matches the input signal. The input seems to freeze on the screen and you can see its shape. You can precisely measure properties of the signal from this line.
Nowadays oscilloscopes are often digital, but cathode ray ones still have a place.
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