Horizontal Deflection
If a voltage which increases linearly, with respect to time, is applied
to the horizontal (X) deflection plates, the spot will move across the
screen at a uniform speed.
Once the voltage is at its maximum, if it then decreases to its starting voltage as instantaneously as possible, the spot will fly back to the start position rapidly.
Because the repetition frequency of this horizontal deflection voltage represents a known period of time, this voltage is known as the timebase voltage.
The second picture shows the effect of applying voltages to both sets of deflection plates simultaneously. In most case, changes in the voltage under investigation will be much faster than shown here and 'persistence' of the fluorescent coating on the screen will give the illusion of a continuous line being 'painted'.