An etheric beam locator was a device frequently carried by the Doctor. It was presumably used to locate etheric beams, and was also useful for detecting ion-charged emissions.
When captured by Kaleds on Skaro, the Fourth Doctor had an etheric beam locator in his pocket. (TV: Genesis of the Daleks) He later had it in London in 1889 (TV: The Talons of Weng-Chiang) and on Hephastos. (AUDIO: Babblesphere)
Behind the scenes Edit
The item that doubled as the Fourth Doctor's etheric beam locator in Genesis of the Daleks and The Talons of Weng-Chiang was (in real life) a Varian Klystron VA220 valve used in microwave transmission and oscillator equipment, such as television transmissions and radar technology. 
The parodic Comic Relief story The Curse of Fatal Death had its version of the Master outfitted with etheric beam locators, which were "Dalek bumps" placed in such a way that they appeared to give the Master "breasts".