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.[1] The alternative name 'harmonica bug' refers to the fact that such devices were originally activated using the tone produced by a harmonica.[2]
Design of infinity transmitters has varied, according to developments in telephone systems. In some instances, the bug is activated after the target answers and hangs up their phone. In countries where there is a delay between connection and the first ring, the bug can be activated before the target phone rings, so that the infinity transmitter essentially 'answers' the call. In more advanced systems, the transmitter can be placed in a parallel telephone line to prevent the victim's phone line remaining engaged.[2]