Combadge

The combadge is a combination of a badge and communicator, worn by Starfleet personnel on their uniforms during the 24th century. Variations of this form of device have been used on uniforms of other cultures at similar levels of technology also.

Combadges are used for on board ship communication with other Starfleet personnel, when using the internal communication system is not possible or impractical, for accessing the on board computer when not in an area that the computer is monitoring, for ship-to-shore communications, and for direct communication to another combadge. Transporters use the combadge as a locator device to lock-on to its bearer, thus making transport faster and more accurate. Combadges are configured to act as universal translators. 

Due to the size of the combadge, its working range is limited to 500 km, although a starship can boost the signal to bridge larger distances. The combadge can be opened for maintenance purposes or to be used as a beacon. 

In emergency situations, a combadge can be modified for use in other applications. It can be converted into a subspace distress beacon, or the tiny power cell can be extracted for other uses. Lieutenant Worf used it to power a home-made field generator for several seconds. If the casing of the combadge is ever cracked, an emergency distress signal is emitted. The Starfleet combadge can be deactivated by tapping it twice in rapid succession. (note: the EMH of USS Voyager used this technique not to deactivate his combadge, but to gain the attention of the ship's crew )

As a symbol of loss of membership in Starfleet, personnel that have resigned or have been relieved of duty must turn in their combadges. Taking off one's combadge constitutes taking off one's uniform. 

Background Information
The Starfleet insignia pin first used in would later become a combadge, as Lieutenant Richard Castillo can been seen pressing his and speaking into it in TNG: "Yesterday's Enterprise." The combadge with the rectangular back was first seen in the Star Trek: Deep Space Nine episode "The Search, Part I", before being used continually in the series and Star Trek: Voyager. It also appeared in the four Next Generation movies.

There have been three possible future combadges used in Star Trek. In, the combadge comprised the delta shield and rectangular strips behind it signifying rank, doing away with rank pips. The second future combadge seen comprises a see-through delta shield and two gold, vertical rectangles behind it. This badge has been seen in, , and. Also seen in Voyager is a 29th century combadge. This is a 'winged' arrowhead, similar in style to the insignia found on a 24th century starship hull. It has been seen in and.