Over exposed - not enough ion

An ion chamber is a little capsule of gas, about the size and thickness of a business card. When the gas is exposed to X-rays, it ionizes. That means some of the gas atoms have electrons knocked off them, creating an ion. The ion chamber has a high voltage applied across it.
Because the ion now has a positive charge (fewer electrons), it rushes toward the negative side of the chamber, created by the high voltage. A small electrical current is produced. We can take that current and measure it. When the ion chamber receives enough xrays, ie enough ionization, the measuring circuit tells the X-ray generator, "that's enough X-ray thanks" (measuring circuits don't say words, but you get the drift) and switches off the X-rays.

This is actually a very good thing. If you're having an X-ray taken, you want that X-ray to turn off when the X-ray film is properly exposed. That means the radiologist (doctor) gets a good view of your broken collar bone, lungs, skull or whatever, and you don't get ionized too much. Yep, you ionize too when exposed to X-rays. But don't worry. The amount of ionization is much, much less than the cosmic rays you're exposed to while flying in a plane.
So to make a good exposure, the ion chamber is placed in the X-ray beam, and it's adjusted to turn off the X-ray generator when it receives enough X-rays to make a good exposure on the film. The X-ray beam starts at the X-ray tube, then passes through you, then the ion chamber, then the film.

So I was called to investigate why films in this particular room were always dark, ie too much X-ray. You're way ahead of me here. You've probably already guessed that the ion chambers weren't working. Why was that? Because someone had unplugged the circuit that measures the current from the ion chamber. It wasn't me I promise.

So plugging the circuit back together fixed the problem.

Next, Xrays on de move, wit de fault.