The Earthmen Detector was an object which the Daleks created to help detect hiding humans. It was equipped onto the Dalek casing through a band encircling the base unit.
When the Daleks tested the prototype, the device went off at the exact moment that a cat passed it, making them think that it was defective. The Daleks left to destroy the useless artefact, as the First Doctor, Ian and Barbara came out of hiding. (COMIC: The Daleks Are Foiled)
