It's pretty simple, actually. You use it and it tells you how many Rads you have. As long as you keep your Rad count under 100, you should be fine. Go above 100 and you're in some trouble. The worst part is you won't even know you've been sucking up Rads until a few days later. Get over 1000 and you're cooked - unless you get a hold of a whole lot of Rad-Away.”— Jake
The Geiger counter is a Wattz Electronics C-Radz model Geiger counter. It detects the presence and strength of radiation fields.
Geiger counters make a familiar ticking sound when radiation is detected by it. The more radiation there is the faster and louder the ticking becomes. Useful for walking in areas with high amounts of radiation, and swimming and/or wading in irradiated bodies of water, which can both be lethal or hazardous to health. The Geiger counter has a limited amount of charges, which are depleted as it is turned on and off. It can also be used in one of the two slots to read one's rad-count.
One can be purchased in the Cathedral from the shopkeeper. Another can be found in the General Store in the Hub and an additional two bought from Jake in Old Town and Killian in Junktown. There is another one in the 3rd level of The Glow in the locker room to the left of the elevator.
One can be found in Vault City, in the lockers on the 3rd floor of Vault 8. There is also a geiger counter in an easy-to-lockpick locker in the greeting office at the entrance of the city. The door is locked, so lockpicking is needed to access it. There is also a variety of valuable loot there, too. Another can be found on a bench in the basement of New Reno Arms.
- In Fallout 3 and Fallout: New Vegas, it is shown that the Pip-Boy 3000 & the Pimp-Boy 3 Billion have a built-in Geiger counter.
- In Van Buren, it is a quest item in Get them a Geiger Counter, to solve the problem of the radioactive lizards, and would help the salvagers steer clear of high-rad areas in Denver.
- Geiger counter on Wikipedia.