In oil droplet experiment, x-ray makes the air molecules negatively charged. How does that work? X-ray carries high energy and ionizes air, doesn't that make air positively charged?
I recently did this lab and the answer I got to this question was:
The x-rays ionize the air molecules or gas in between the capacitors. The free electrons then fly off and attach them selves to the oil drops. Alternatively, you could use an electron beam (same effect).
According to this link, the x-rays ionize the gas molecules in the apparatus, not the oil drops directly. So yes, you are right, the air is positively ionized. The newly freed electrons from the gas then adhere to the oil droplets (probably due to an induced dipole moment), producing negatively charged oil.