Etheric beam locator

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 also had it in Victorian London. (TV: The Talons of Weng-Chiang)

When faced with a bomb, the Sixth Doctor searched his pockets for an etheric beam locator. (AUDIO: The Condemned)

The Doctor later searched for a type 7 enhancer for his etheric beam locator. (AUDIO: The Raincloud Man)

While in Hexen Bridge, the Seventh Doctor had a sub-etheric beam locator in his pocket. (PROSE: The Hollow Men)

On Hyspero, Sam and Gila traded a dismantled and useless etheric beam locator for some bread. (PROSE: The Scarlet Empress)

While on Janus Prime, the Eighth Doctor had a sub-etheric beam locator in his pocket. (PROSE: The Janus Conjunction)

Behind the scenes
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 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".