Without any further actions, all electrons would hit the center of the screen. We can deflect the electron beam by applying a magnetic field. This magnetic field is produced by deflection coils. There are vertical deflection coils, moving the electrons up and down, and horizontal deflection coils moving the beam sideways.

The TV picture is refreshed 25 times per second (PAL format). So the frequency of the vertical deflection coil current is 25Hz. The period time is 40ms. In that interval 625 horizontal lines are written. So the period time of the current in the horizontal deflection coils is 64us. This is also referred to as the scan time. It corresponds to a frequency of 15625Hz. This is the high pitched whine we someting hear coming from a TV set.