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 / reality) a Varian Klystron VA220 valve used in microwave transmission and ocsillator equipment, such as television transmissions and RADAR technology. Source - http://www.decadecounter.com/vta/tubepage.php?item=20&user=O

The non-canonical 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".