Question

$\left(i\right)$ Why is a cathode ray tube evacuated to a low pressure?
$\left(ii\right)$ What happens if the negative potential is changed on a grid?

Answer 1

$\left(i\right)$ The cathode ray tube is evacuated to a low pressure so that the electrons can move without being obstructed by air molecules.
$\left(ii\right)$ The variation of the negative potential on the grid affects the number of electrons reaching the anode and striking the screen changed which changes the brightness of pattern on the screen.