The apparatus which converts current signals to pneumatic and pneumatic to current signals is called a:

The apparatus that converts current signals to pneumatic and pneumatic signals back to current signals is accurately identified as a transducer. In the context of refrigeration and control systems, transducers are essential components that facilitate the communication between different types of systems.

Transducers work by converting energy from one form to another. Specifically, in this case, when a current signal (which is an electrical signal) is transformed into a pneumatic signal (which utilizes air pressure to convey information), the transducer enables the interaction between electronic control systems and pneumatic devices. Conversely, when it converts pneumatic signals back into electrical signals, it allows sensors and control systems, which primarily operate on electrical signals, to interpret the information derived from mechanical systems.

The other options do not perform this dual function of converting between electrical and pneumatic signals. A transformer is specifically meant for changing the voltage levels of electrical signals, while inverters convert direct current (DC) to alternating current (AC). A converter typically refers to changing one form of electrical input to another type of electrical output, but it does not encompass the broad functionality of converting between pneumatic and electrical signals. Thus, the correct term for the device described is indeed transducer, as it captures the critical role of signal conversion across different mediums.