Final answer:
A positioner is the device installed between a control valve and the air supply header.
Step-by-step explanation:
The device installed between a control valve and the air supply header is called a positioner. A positioner is a control device that is used with control valves to accurately position the valve stem, based on the control signal received from the process control system. It ensures that the valve is in the correct position to regulate the flow of air or fluid.