|First Appearance||A Study in Pink|
Afterwards, Murdoch explains to Henry how it works, "As the wearer walks, a magnet slides back and forth inside of a wire coil inducing an electrical current. That current is then converted to voltage...Now, as the voltage increases on either side of this gap, the pressure builds and builds and builds until...(clap)... a tiny spark!"
But, Henry asks what happens when the quarry stops moving? "Well then, the transmission ends until movement resumes."
But, "what happens if they've arrived, but we're still a block away?" " That could be a problem. Well, that would necessitate a third receiver unit in order to triangulate the... Oh, Henry!"