Which of the following is NOT part of automatic processing of x-rays?

Automatic processing of x-rays involves a series of steps that are typically facilitated by a machine designed to streamline the development, fixing, and drying of x-ray film. In this context, the components essential for this automated processing include a chemical and water tank, developing fluid, and a fixing tank.

The temperature control unit is not considered a physical part of the automatic processing machine itself but rather a regulating mechanism that ensures the temperature of the chemicals remains at optimal levels for development. While maintaining temperature is critical for proper processing, it operates as part of the environment or system rather than being a component that directly interacts with the film. This distinguishes it from the other options that directly contribute to the chemical processes of developing and fixing the film itself.