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." (TV: The Daleks)

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

The First Doctor had such a device, but it went broken during his first travel with Ian and Barbara. (TV: An Unearthly Child)

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")

The equipment of the soldiers of the Combined Galactic Resistance was provided with such technology. It helped to diagnose a radiation leak as the cause of malfunctioning of a Dalek. (TV: Into the Dalek)