A servo was a multi-function device employed by Gary Seven that resembled a pen. (TOS: "Assignment: Earth")

The servo had several functions, and served as:

• A communicator, which allowed Gary Seven to communicate with the Beta 5 computer and initiate transport.
• A personal defense mechanism. It had both a stun setting, which made its target go into a hypnotic state open for suggestion, and a kill setting.
• An electronic manipulation device, which Gary Seven used to deactivate the force field keeping him in the brig of the USS Enterprise.
• A mechanical manipulation device, with the ability to serve simple purposes such as unscrewing screws, and locking and unlocking doors.