The four devices that make up a control loop are the primary element, the ________, the controller, and the final control element.

In a control loop, after the primary element, the second device is typically the transmitter. The primary element detects the process variable, such as temperature, pressure, or flow, and converts this physical measurement into a signal that the transmitter can process. The transmitter then communicates the signal to the controller, which analyzes it against the desired set point and makes necessary adjustments.

The role of the transmitter is crucial as it ensures that the information from the primary element is accurately sent to the controller, allowing the entire control loop to function efficiently. This relationship forms a critical pathway in which data is monitored and managed, so that the final control element can act to regulate the process effectively.

The other terms provided do not serve as the second device within the control loop structure. While a set point refers to the desired value for the process variable, and an anchor point may relate to reference values or positioning, they do not define a component that continually interacts with the primary element or transmits data back to the controller for processing. Thus, the transmitter is indeed the correct choice in the context of control loop components.