Infinity transmitter

An infinity transmitter (also known as a harmonica bug) is a surveillance device used to covertly monitor conversation in a room through a telephone line. Its name derives from the fact that, by using a telephone line as a transmitter, it can work at an infinite distance, unlike other bugging devices that have only a finite signal range. The alternative name 'harmonica bug' refers to the fact that such devices were originally activated using the tone produced by a harmonica.