Final answer:
The automatic exposure device under the x-ray table is the ionization chamber, used for measuring radiation exposure through ionization of gas and collecting the resulting current.
Step-by-step explanation:
The automatic exposure device that is located immediately under the x-ray table is the ionization chamber. This chamber uses the principle of ionization to measure the dose of radiation efficiently and accurately. An ionization chamber is a device filled with gas that becomes ionized when exposed to radiation; the ions are collected and the resulting current is measured to determine the level of exposure. Other devices such as the scintillation camera and photomultiplier are also relevant in the context of radiation detection but serve different purposes. A scintillation camera detects radiation and converts it to light, which is then amplified by the photomultiplier to provide an image or measurement.