The field coil was a form of technology designed to transmit subspace emissions, which functioned as key components in starship and space station operations, including the sensors, warp drive and communications systems. (TNG: "Force of Nature" ) After entering V'ger, Spock discovered what he described as "a kind of plasma-energy conduit – possibly a field coil for a gigantic imaging system," where he discovered a series of images depicting objects encountered by V'ger. (Star Trek: The Motion Picture)