A Wraith relay device is used by the Wraith to locate the locations of individuals, either their own warriors or other people. Currently two versions of the transmitter are known.
The first transmitter had the size of a pendant and was used as such by Teyla Emmagan, who got it from her father. She lost it as a child but later it was found by Major John Sheppard, whose touch unknowingly activated it. During future missions the transmitter sent informations to relay devices which informed the Wraith, causing them to attack Sheppard's team on several missions. The true nature of the device was finally discovered by Dr. Rodney McKay. Afterwards the Atlantis expedition used the device to lure some Wraith into a trap and were able to capture one. (SGA: "Rising", "Suspicion")
Another transmitter was discovered a while later on M7G-677. This transmitter had the form of a large bracelet and was worn by the corpse of a Wraith warrior, indicating that perhaps all warriors wear such a device. (SGA: "Childhood's End")