Which one of the following symbols represents a pressure switch?

The symbol that represents a pressure switch is typically a square with an 'S'. The use of this specific symbol is standardized in various schematics to indicate components that control pressure levels.

In many electrical diagrams and control schematics, the square represents a control device, and the letter 'S' specifically denotes a switch. Pressure switches monitor the pressure within a system and open or close electrical contacts depending on whether set pressure levels are surpassed or not. This is essential for maintaining safe operating conditions in a variety of systems.

The other symbols mentioned do not correctly correspond to a pressure switch: the circle with a dot often represents a light or indicator, the rectangle with a diagonal line may indicate an output or an actuator, and the triangle is frequently used to symbolize caution or warnings rather than switching devices. Thus, the square with an 'S' is the most accurate representation for a pressure switch in schematics.