A stethoscope was an instrument that amplified sound, allowing the wearer to better hear the sounds emanating from the site on which the bell of the device was placed. Though typically meant for medical practice — as when Harry Sullivan and Martha Jones examined the Doctor's hearts (TV: Robot, Smith and Jones), it could be used in other, non-medical situations. The Doctor, for instance, frequently used it to amplify the sound that mechanical or electronic devices made, (TV: Fury from the Deep, The Stolen Earth) or to eavesdrop on a conversation in an adjacent room. (AUDIO: Storm Warning, TV: Partners in Crime) The Doctor found it such a useful device that many of his incarnations, including Second, Fourth, Eighth, Tenth and Eleventh carried one.
Instances of use
Tina the cleaner's lodger was a medical student. Jackie took the lodger's stethoscope while she was asleep and Rose then used it to check that both of the newly-regenerated Tenth Doctor's hearts were working. When Rose later checked, only one heart was beating. (TV: The Christmas Invasion)
After being checked into the Royal Hope Hospital, the Tenth Doctor was examined by Martha Jones, who used her stethoscope to listen to the Doctor's heart. When she did, she found he had two hearts. (TV: Smith and Jones)
By the Doctor
On several occasions, the Doctor used the stethoscope as a non-medical tool.
The Second Doctor used a stethoscope to listen to the pipes emanating from the Euro Sea Gas plant. He at first thought he detected a heartbeat in the pipes, but dismissed it as the sound of a pump. In fact, he had heard the stirrings of organic life —that of the weed creature. (TV: Fury from the Deep)
The Eighth Doctor carried a stethoscope he had "borrowed" from Arthur Conan Doyle, which he needed to return to Doyle before the writer stopped practising medicine. The Doctor used it to eavesdrop whilst aboard the R101. (AUDIO: Storm Warning)
When the Earth was taken to the Medusa Cascade, Harriet Jones, Torchwood and Mr Smith all worked together to send a signal to Martha's phone. When it rang, the Tenth Doctor answered to find it only to be a signal, which he then followed by using his stethoscope. (TV: The Stolen Earth)