A thermostat with a single mercury bulb which controls a single stage heat, single stage cool unit is a ... control

A thermostat with a single mercury bulb designed for controlling a single stage heating and a single stage cooling unit is characterized by its functionality to switch between two different states—heating and cooling. This is why it is categorized as a single pole double throw (SPDT) control.

In an SPDT configuration, there is one input or "pole" that can connect to one of two outputs or "throws." The mercury bulb thermostat enables this setup by allowing the system to either close the circuit for heating or cool the circuit for cooling, effectively controlling which operation is activated based on the temperature reading.

The distinction of being SPDT is crucial because the device must be capable of switching between two different conditions (heating and cooling). Other types of controls do not have the same capability. For example, a single pole single throw (SPST) control would only allow for one output— either fully on or fully off—without the switching capability needed for alternative functions. Similarly, double pole configurations, like DPST or DPDT, involve more complex connections that do not directly pertain to this simple heating and cooling application. Therefore, the correct classification of the thermostat's control is SPDT, aligning perfectly with the device's operational purpose.