A standard that is still in use today, the 3.5 mm or micro size phone jack was initially created in the 1950s as two-conductor connectors for earpieces on transistor radios.
Sony EFM-117J radio (which was launched in 1964) made this nearly half-sized of the original popular it is still frequently used in portable applications today.
The Sony Walkman lacked a speaker of its own, unlike earlier transistor radios, the three-conductor version quickly gained popularity.