Thank you Jamon! I'm not quite sure how the signals in deflection plates work actually... 
could you clarify for me!
Cheers!
Sure!
Okay, so let's just assume we are working with electric field deflection (you can apply the same idea to magnetic). The idea is that by controlling the electric field strength and direction, we can change the path of the electron beam. In this case, this would be done by applying different voltages to the deflection plates
Let's say we can apply a voltage between 100V and -100V to the plates (same max magnitude, just opposite polarity) to a pair of plates controlling vertical deflection. 0V would mean the electron beam is unaffected, and travels straight. If we apply 100V, the beam might (for example) be deflected such that it hits the very top of the screen. 99V means it hits JUST below that. 98V, just below that again. All the way to zero where we are back in the centre. Then, -1V, just below the centre. And the process continues, all the way to -100V where the beam hits the bottom of the screen.
If we do this for both horizontal and vertical deflection, we can hit ANY point on our two dimensional screen. The top right corner might be 100V applied to vertical plates, 100V applied to horizontal plates. A point in the bottom right would be -100V vertical, but still 100V horizontal. We adjust the voltages of each pair to move up/down, or left/right.
We can do creative things with this ideas (again, just assume electrical is used for everything to keep things numerically simple). For example, in an Oscilloscope, the horizontal plates have a time varying voltage. We start by applying -100V, and the beam is at the very left. We gradually increase this to 0, and then to 100, to cause the beam to sweep across the screen. Then, when we hit 100V, we immediately reset to -100V and start the sweep again (for prospective electrical engineers, this signal would be called a
sawtooth wave).
So we have an electron beam sweeping across the middle of the screen (maybe once a second). If we then connect the vertical deflection plates to some source we want to measure (for example, an electrode connected to the chest to measure heartbeat), that means the electron beams vertical position is dictated by what we are measuring. Thus, we then have your typical cardiograph, with the heartbeat visible as it fluctuates over time! The beam sweeps across, and jumps up and down as it sweeps in correspondence with the heartbeat.
Television screens are more complex, but basically, both the vertical and horizontal deflection plates are time varying. The periods are set up such that the beam does a sweep across the top row of the screen, then the next row in the reverse direction, then the next row, etc etc. Remember Donkey Kong? How you'd go all the way to the right, then jump, then all the way to the left, then jump, etc? That's what happens here (roughly speaking). It's a little tougher to picture, but let's say we start in the top left corner:
Horizontal = 100V, Vertical = 100V
Then we want the 'phosphor dot' to its right:
Horizontal = 99V, Vertical = 100V
We keep sweeping to the right until we get to the top right corner:
Horizontal = -100V, Vertical = 100V
Then we shift down:
Horizontal = -100V, Vertical = 99V
Then we begin sweeping left:
Horizontal = -99V, Vertical = 99V
And the process repeats in this fashion
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