English: A tiny crystal radio receiver built into a finger ring by a young radio enthusiast around 1922, from a radio magazine. Radio broadcasting started around 1920, and in the early 20s radio listening exploded into a hugely popular new high-tech pastime. The most popular type of receiver was the simple crystal radio, a passive receiver that ran off the power of the station's radio waves; these were homemade by the millions. It consisted of a length of wire aerial, a tuning coil, a small crystal of a mineral such as galena which rectified the radio signal to extract the audio modulation, and earphones. During the "radio craze" of the early 20s hobbyists built tiny crystal radios like this as novelties into pipes, matchboxes, watches, and umbrellas. The crystal detector can be seen on the ring in closeup photos. This receiver may not have had a tuning coil and may just consist of a detector and earphones; such simple crystal radios just pick up the most powerful station in the vicinity.

Caption: "A FINGER-RING RECEIVER. This is claimed to be the smallest set made. Do our readers know of one smaller?"