When the liquid level in the tank reaches the float switch, it pushes the float up. A magnet located inside of the float then causes the contacts of the internal reed switch located inside of the stem to close. When the contacts close it completes the circuit connecting power to the alarm. It works the same way for low level alarms, when the float drops down it completes the circuit activating the alarm.
These alarms can be activated with standard single level float switches or made to order multi level float switches. The float switches come in different materials and sizes to adapt to almost any tank. The alarms are offered in various different voltages and are available with audible, visual, or both audible and visual indication.