A Chambers coil was a component of communications systems.
In 2285, Khan Noonien Singh, who had taken over the starship USS Reliant, ordered a message sent to the approaching USS Enterprise, claiming that their Chambers coil was overloading their comm system, as an excuse as to why they could not engage visual communications. This ruse was discovered by Captain Spock, however, who was able to determine that the Reliant's coil emissions were normal. (Star Trek II: The Wrath of Khan)
In 2370, while attempting to escape from the Kellerun on the surface of T'Lani III, Julian Bashir attempted to repair a comm system to reach the T'Lani, under instruction from Chief O'Brien. Part of this process included resetting the actuators on the Chambers coil to switch to a higher carrier frequency. (DS9: "Armageddon Game")