Banana connector
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.
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.
Individual banana plugs and jacks are commonly color-coded red and black, but are available in a wide variety of colors.
The 3⁄4 inch spacing originated on General Radio test equipment during the 1920s, and their type 274-M dual-plug is a notable example from that period.
One type is 2.6 mm in diameter and about one third the length of the standard connector, and is commonly used in model railway applications.
Another design used an actual split pin of plated steel, the parallel ends of which formed the plug tip.
It featured a machined brass component with an internal bore of just over 1⁄8 inch, a transverse slot to accommodate the eye end of the split pin, and an external screw thread to engage the coloured hard plastic cap, tightening of which trapped the stripped end of the wire in the eye of the split pin to establish a secure connection.
Wander plugs generally would not inter-mate successfully with banana sockets, and attempts to do so by spreading the brass tip often ended with its fracture.
The hazards include electric shock, electrocution, burns from accidental short circuits, and damage to the attached equipment.
These plugs have fixed or sliding sheaths or other devices to protect the user from accidental contact with live conductors, but are still largely compatible with the original design.
When inserted, the sheaths of the plug and socket overlap fully, ensuring that no live metal is exposed.
A typical design is now required (IEC 61010) on multimeter test leads and several other types of measurement and laboratory equipment.
In this design, the metal banana plug is entirely sheathed in a plastic tube and inserts into a deeply recessed jack in the multimeter.
In most European countries, the standard mains power receptacle will physically accept banana and even US-style "double banana" plugs (the standard US pin spacing of 3⁄4 inch (19 mm) is close enough to the mains plug spacing of about 19 mm, and the pin diameter is also compatible), leading to a risk of electrical shock.
