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."

20th century humans used Geiger counters, and Ian Chesterton and Barbara Wright were both familiar with the machine.

The Daleks made use of one, and the Doctor, Susan Foreman and Ian 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")