This electronic level uses two LED indicators instead of an air bubble. If the surface is tilted to the right, one LED lights; if it's tilted to the left, the other LED lights. When the surface is level, both LEDs light. It uses two unidirectional mercury switches, SI and S2. The unidirectional mercury switch has one long electrode and one short, angled electrode.
The pool of mercury "rides" on the long electrode and makes contact between the two electrodes if the unit is held in a horizontal position.