The sensor is called an induction loop, and it’s basically a big loop of wire that has electricity running through it.
这种感测器叫感应线圈,基本上就是一大圈的电线内有电流经过。
英语百科
Induction loop
An induction or inductive loop is an electromagnetic communication or detection system which uses a moving magnet to induce an electric current in a nearby wire. Induction loops are used for transmission and reception of communication signals, or for detection of metal objects in metal detectors or vehicle presence indicators. A common modern use for induction loops is to provide hearing assistance to hearing-aid users.