English: A tuning inductor used in the earliest crystal radio receivers during the wireless telegraphy era between 1900 and 1920. It consists of an air-core transformer made two coils of many turns of fine wire one inside the other. The stationary primary coil is inside the box (right) while the secondary coil (left) is mounted on a rack and can be slid in or out of the primary. This was called a Navy-style "loose coupler". The primary was connected to the wire antenna and ground, while the secondary was connected to the radio's crystal detector. To tune the radio to different stations the switches were adjusted to select different taps on the coil. If interference from a station nearby in frequency to the desired station was encountered, the secondary could be slid further out of the primary, which reduced the mutual inductance ("coupling"), sharpening the selectivity to reject the interfering signal.