Banana plugs

A banana connector (commonly banana plug for the male, banana socket or banana jack for the female) is a single-wire (one conductor) electrical connector used for joining wires to equipment. The term 4 mm connector is also used, especially in Europe, although not all banana connectors will mate with 4 mm parts, and 2mm banana connectors exist. Various styles of banana plug contacts exist, all based on the concept of spring metal applying outward force into the unsprung cylindrical jack to produce a snug fit with good electrical conductivity. Common types include: a solid pin split legthwise and splayed slightly, a tip of four leaf springs, a cylinder with a single leaf spring on one side, a bundle of stiff wire, a central pin surrounded by a multiply-slitted cylinder with a central bulge, or simple sheet spring metal rolled into a nearly complete cylinder. The plugs are frequently used to terminate patch cords for electronic test equipment, while sheathed banana plugs are common on multimeter probe leads.

Invention of the plug is claimed by two entities. The Hirschmann company claims it was invented by Richard Hirschmann in 1924. A competing claim is made for the General Radio Company (Genrad), which stated "1924: GenRad developed banana plug - replaces pin plugs, this spring-loaded connector technology..." and that it was "introduced in this country [the US] by GR in 1924".

The original plug consists of a cylindrical metal pin about 20 millimetres (0.79 in) long. This pin length is still common in Europe and other parts of the world. However other sizes have emerged, such as 15 millimetres (0.59 in) pins, which can commonly be found in the US. Other lengths between 11 millimetres (0.43 in) and 25 millimetres (0.98 in) are less common.

The pin's diameter is nominally 4 millimetres (0.16 in). The pin has one or more lengthwise springs that bulge outwards slightly, giving the appearance of a banana. Taking the springs into account, the actual diameter of a banana plug is typically a bit larger than 4 mm when not plugged in. When inserted into a matching 4 mm socket the springs press against the sides of the socket, improving the electrical contact and preventing the pin from falling out. The other end of the plug has a lug connector to which a length of flexible insulated equipment wire can be attached, which is either screwed, soldered, or crimped into place. On banana plugs for laboratory use, an insulating plastic cover is usually fitted over this rear end of the connector, while banana plugs for speaker connections often use uninsulated metal bodies.

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