Geiger counter

A Geiger counter was a piece of technology involving radiation that made a ticking noise. The First Doctor considered it to be advanced technology. It included a needle on a scale that could reach a "danger point."

The Daleks made use of one, and the Doctor, Susan and Ian Chesterton once encountered it on Skaro. It showed them the excess radiation there, which resulted in a jungle being turned to stone, the soil becoming barren, and sickness among the three. (TV: "The Survivors")