s with GU10
bi-pin twist-lock mount with double contact B22d bayonet mount with double contact B22d bayonet mount, and corresponding socket with sprung connectors The bayonet light bulb mount is the standard fitting in many former members of the
British Empire including the
United Kingdom,
Australia,
India,
Ireland, and
New Zealand,
Hong Kong, as well as parts of the
Middle East and
Africa (although not
Canada, which primarily uses
Edison screw sockets along with the
United States and
Mexico). The standard size is B22d-2, often referred to in the context of lighting as simply BC or B22. Older installations in some other countries, including
France and
Greece use this base. First developed by
St. George Lane Fox-Pitt in the UK and improved upon by the
Brush Electric Company from the late 1870s onward, standard bulbs have two pins on opposite sides of the cap; however, some specialized bulbs have three pins (cap designation B22d-3) to prevent use in domestic light fittings. Examples of three-pin bulbs are found in mercury street lamps and fireglow bulbs in some older models of
electric radiative heater. Older railway carriages in the UK also made use of a 3 pin bulb base to discourage theft. Bayonet cap bulbs are also very common worldwide in applications where vibration may loosen screw-mount bulbs, such as
automotive lighting and other small
indicators, and in many
flashlights. In many other countries the
Edison screw (E) base is used for lighting. Some bulbs may have slightly offset lugs to ensure they can be only inserted in one orientation, for example the 1157 automobile tail-light which has two different filaments to act as both a tail light and a brake light. In this bulb each filament has a different brightness and is connected to a separate contact on the bottom of the base; the two contacts are symmetrically positioned about the axis of the base, but the pins are offset so that the bulb can only be fitted in the correct orientation. Newer bulbs use a
wedge base which can be inserted either way without complication. Some special-purpose bulbs, such as infra-red, have 3 pins 120 degrees apart to prevent them being used in any but the intended socket. Bayonet bases or caps are often abbreviated to BA, often with a number after. The number refers to the diameter of the base (e.g., BA22 is a 22 mm diameter bayonet cap lamp). BA15, a 15 mm base, can also be referred to as SBC standing for small bayonet cap. The lower-case letter
s or
d specifies whether the bulb has single or double contacts. The entries from the table below pertain to
IEC 60061 "Lamp caps and holders together with gauges for the control of interchangeability and safety" Of these, only the BC (BA22d, often abbreviated as B22) is widely used in homes. Formerly, some linear fluorescent lamps in the UK used BA22d end caps, owing to material shortages arising from the Second World War, which prevented the development of the bi-pin cap design that was becoming commonplace elsewhere in the world; notably, in the United States. Production of these lamps continued until the early 1980s, although manufacturers had produced adaptors that permitted bi-pin lamps being used in older luminaires (equipped with bayonet lamp holders) since the 1960s. The BA20d (sometimes called a Bosch fitting) was once a common automotive (twin filament) headlamp fitting but has largely been superseded by more modern, higher-rated H-series sockets and is only used for some lower-powered applications such as combined automotive tail and stop lamps. In Japan, the JIS C 8310 “hook ceiling” bayonet mount is quite common. It is designed to both provide power and carry the weight of a lamp. A similar concept existed in BS 7001 as the slide-in “luminaire-supporting coupler” (LSC), but its prominence is unknown. ==Other uses==