Jacket -- (1) A rubber, plastic or synthetic covering applied over the primary insulation, braids, shields, cable components, or over the cable itself. (2) In fiber optics, a covering, frequently plastic, over a fiber, bundle of fibers, or cable which protects against the environment.

Joule (J) -- A unit of energy or work. The absolute joule is equal to 10 million ergs. The internal joule is equal to the work required to maintain a current of one ampere for one second in a resistance of one ohm.

Jumper -- A short length of conductor used to make a connection between terminals, around a break in a circuit, or around an instrument. It is usually a temporary connection.

Jumper Cable -- A short flat cable interconnecting two wiring boards or devices.

Jumper Wire -- PVC insulated copper conductors twisted together and used for cross-connecting on distributing frames, usually 22 AWG.

Junction -- A point in a circuit where two or more wires are connected.