As the float follows the level of liquid in the vessel, the magnet also moves up and down. The magnet activates the reed switch when they're both at the same level, providing an "on" or "off" signal.

WIKA’s model UTN top-mounted level indicator consists of a measuring chamber, a float with guide rod and a magnetic system. Mounting onto the vessel is made via appropriate process connections ...

Following successful third-party evaluation by the global organization Exida, Mobrey Horizontal Magnetic Float Level Switches have the required Failure Modes, Effects and Diagnostic Analysis (FMEDA) ...