A phone connector is a family of cylindrically shaped electrical connectors primarily for analog audio signals. Invented in the late 19th century for telephone switchboards, the phone connector remains in use for interfacing wired audio equipment, such as headphones, speakers, microphones, mixing consoles, and electronic musical instruments (e.g. electric guitars, keyboards, and effects units). A male connector (a plug), is mated into a female connector (a socket), though other terminology is used. Plugs have two to five electrical contacts. The tip contact is indented with a groove. The sleeve contact is nearest the (conductive or insulated) handle. Contacts are insulated from each other by a band of non-conductive material. Between the tip and sleeve are zero to three ring contacts. Since phone connectors have many uses, it is common to simply name the connector according to its number of rings:
The sleeve is usually a common ground reference voltage or return current for signals in the tip and any rings. Thus, the number of transmittable signals is less than the number of contacts. The outside diameter of the sleeve is 6.35 millimeters (1⁄4 inch) for full-sized connectors, 3.5 mm (1⁄8 in) for "mini" connectors, and only 2.5 mm (1⁄10 in) for "sub-mini" connectors. Rings are typically the same diameter as the sleeve.
Other terms The 1902 International Library of Technology simply uses jack for the female and plug for the male connector. The 1989 Sound Reinforcement Handbook uses phone jack for the female and phone plug for the male connector. Robert McLeish, who worked at the BBC, uses jack or jack socket for the female and jack plug for the male connector in his 2005 book Radio Production. The American Society of Mechanical Engineers, as of 2007, says the more fixed electrical connector is the jack, while the less fixed connector is the plug, without regard to the gender of the connector contacts. The Institute of Electrical and Electronics Engineers in 1975 also made a standard that was withdrawn in 1997. The intended application for a phone connector has also resulted in names such as audio jack, headphone jack, stereo plug, microphone jack, aux input, etc. Among audio engineers, the connector may often simply be called a quarter-inch to distinguish it from XLR, another frequently used audio connector. These naming variations are also used for the 3.5 mm connectors, which have been called mini-phone, mini-stereo, mini jack, etc. RCA connectors are differently shaped, but confusingly are similarly named as phono plugs and phono jacks (or in the UK, phono sockets). 3.5 mm connectors are sometimes—counter to the connector manufacturers' nomenclature—referred to as mini phonos. Confusion also arises because phone jack and phone plug may sometimes refer to the RJ11 and various older telephone sockets and plugs that connect wired telephones to wall outlets.
Historical development The original 1⁄4-inch (6.35 mm) version descends from as early as 1877 in Boston when the first telephone switchboard was installed or 1878, when an early switchboard was used for the first commercial manual telephone exchange in New Haven created by George W. Coy.
Charles E. Scribner filed a patent in 1879 to facilitate switchboard operation using his spring-jack switch. In it, a conductive lever pushed by a spring is normally connected to one contact. But when a cable with a conductive plug is inserted into a hole and makes contact with that lever, the lever pivots and breaks its normal connection. The receptacle was called a jack-knife because of its resemblance to a pocket clasp-knife. This is said to be the origin of calling the receptacle a jack. Scribner filed a patent in 1880 which removes the lever and resembles the modern connector and made improvements to switchboard design in subsequent patents filed in 1882.
Henry P. Clausen filed a patent in 1901 for improved construction of the telephone switchboard-plug with today's 1⁄4 inch TS form still used on audio equipment. Western Electric was the manufacturing arm of the Bell System, and thus originated or refined most of the engineering designs, including the telephone jacks and plugs which were later adopted by other industries, including the US military. By 1907, Western Electric had designed a number of models for different purposes, including:
By 1950, the two main plug designs were:
WE-309 (compatible with 3⁄16-inch jacks, such as 246 jack), for use on high-density jack panels such as the 608A WE-310 (compatible with 1⁄4-inch jacks, such as the 242) Several modern designs have descended from those earlier versions:
B-Gauge standard BPO316 (not compatible with EIA RS-453) EIA RS-453: Dimensional, Mechanical and Electrical Characteristics Defining Phone Plugs & Jacks standard of 0.206 in (5.2 mm) diameter, also found in IEC 60603-11:1992 Connectors for frequencies below 3 MHz for use with printed boards – Part 11: Detail specification for concentric connectors (dimensions for free connectors and fixed connectors).
Military variants U.S. military versions of the Western Electric plugs were initially specified in Amendment No.1, MIL-P-642, and included:
M642/1-1 M642/1-2 M642/2-1 M642/2-2 M642/4-1 M642/4-2 MIL-P-642/2, also known as PJ-051. (Similar to Western Electric WE-310, and thus not compatible with EIA RS-453) MIL-P-642/5A: Plug, Telephone (TYPE PJ-068) and Accessory Screws (1973), and MIL-DTL-642F: Plugs, Telephone, and Accessory Screws (2015), with 0.206 in (5.2 mm) diameter, also known by the earlier Signal Corps PL-68 designation. These are commonly used as the microphone jack for aviation radios, and on Collins S-line and many Drake amateur radios. MIL-DTL-642F states, "This specification covers telephone plugs used in telephone (including telephone switchboard consoles), telegraph, and teletype circuits, and for connecting headsets, handsets, and microphones into communications circuits."
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