Sonic screwdriver

The sonic screwdriver is a tool and defensive weapon used by the Doctor. It is part of TARDIS' toolbox.

A sonic screwdriver is a small cylinder, no larger than a conventional screwdriver. It is capable of performing a myriad of different tasks, including technology repair, opening any kind of lock and, as the name suggests, driving screws. The device also exhibits some defensive capabilities.

History
The sonic screwdriver was first used by the Second Doctor. (DW: Fury from the Deep) It was used by subsequent incarnations of the Doctor until its destruction in the hands of the Terileptil. (DW: The Visitation). The Sixth Doctor and the Seventh Doctor didn't use a sonic screwdriver. It was not until his eighth incarnation that the Doctor began using the tool again. (DW: Doctor Who: The TV Movie). The Ninth and  Tenth incarnations of the Doctor have also used the device.

Other Time Lords have also been seen using similar devices. Romana constructed her own sonic screwdriver. The device impressed the Fourth Doctor, and he attempted, unsuccessfully, to swap sonic screwdrivers with her. (DW: The Horns of Nimon) Human companions have also used the Doctor's device in emergencies. (DW: Inferno)

Technology and Functions
The sonic screwdriver is apparently the product of Gallifreyan technology since other Time Lords (i.e. Romana) use the device and understand it enough to construct their own versions of it. The name of the device itself suggests that it functions using soundwaves, although the actual workings of the device have never been explicitly explained.

The different versions of the Doctor's sonic screwdrivers have exhibited different capabilities and uses. The device has been mainly used as a repair tool and as a lock picker. Although it is primarily a tool, it can also be used as a defensive weapon. However, according to the Tenth Doctor, the device cannot be used to wound or kill living things. (DW: Doomsday)



The sonic screwdriver of the Ninth and Tenth Doctor exhibit more capabilities that the device used by the previous incarnations. These include:
 * Detecting and stopping telepathic signals. (DW: Rose)
 * Interfacing with a computer. (DW: The End of the World)
 * Medical scanning and diagnosis. (DW: The Empty Child)
 * Battery charging. (DW: Father's Day)
 * As a lighter. (DW: The Girl in the Fireplace)
 * Blowing doors open. (DW: Doomsday)