A picture of the submarine HMS Affray showing the jumping wire running up from the bow of the ship (right) to the periscope standards, and then back down to the casing at the stern
A jumping wire is a wire cable stretched between the bow and stern of a submarine, via the conning tower or periscope standards.
Its purpose is to allow the submarine to pass under nets and other marine defences, without the obstruction snagging on the vessel's superstructure, the wire causing the net to ride up and over the top of the submarine.
Jumping wires have fallen out of use since the end of World War II, as the wire tends to vibrate at high underwater speeds creating noise that may be detected by an enemy using passive sonar, as well as causing hydrodynamic drag.
The original article can be found at Jumping wire and the edit history here.