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4mm plug
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4mm plug

4mm plug (also called the banana plug) is a single-pole electrical connector widely used in science laboratories for temporarily joining wires to equipment.  It consists of a cylindrical metal pin about 25 mm long, with a diameter of 4 mm, which can be inserted into a matching 4 mm socket to make an electrical contact.  The pin has one or more longitudinal springs that bulge outwards slightly.  These press against the sides of the socket, improving the electrical contact and preventing the pin from falling out.  The curved profile of these springs is probably the origin of the name banana plug.  The other end of the plug has a hole that accepts a length of flexible insulated equipment wire, which is either screwed or soldered into place.  An insulating plastic cover is usually fitted over this end.

The wide end of a 4 mm plug often has a 4 mm hole drilled in it, either transversely or axially, to accept the pin of another 4 mm plug. This type is called a stackable 4mm plug.