Which element is essential for the operation of a foam extinguisher when under pressure?

The essential element for the operation of a foam extinguisher, particularly when under pressure, is compressed air. Foam extinguishers work by using a combination of foam concentrate and water, which is then mixed and pressurized with air to create a foam that can effectively suppress fires. The compressed air helps to expel the foam from the nozzle, allowing it to blanket the fire and cut off the oxygen supply, which is crucial for extinguishing flames.

Understanding the mechanics of how foam extinguishers operate reveals that without the presence of compressed air, the foam would not be dispensed effectively. The pressure created by the compressed air helps to form the foam and propel it towards the burning material, which is essential for effective firefighting.