Physics Glossary – J

Josephson Junction

Consists of two superconductors separated by a thin insulated layer, 1 to 2 mm thick. At normal temperatures a small current can flow between the conductors but at low temperatures the junction has no resistance. If a magnetic field or current is increased past a critical value then the zero resistance is lost and the cycle starts again. This means that the junction can be used to measure very weak magnetic signals or as extremely fast switched in supercomputers.

Joule-Thomson Effect (Joule-Kelvin Effect)

When a gas expands through a small hole this causes a change in temperature, the change is proportional to the pressure difference across the hole.

Joule’s Law

States that the rate at which heat is produced in any part of an electric circuit is measured by the product of the square of the current into the resistance of that part of the circuit.

Joint European Torus (JET)

The Joint European Torus (JET), situated in Oxfordshire at the Culham Science Centre, is the flagship experiment in the current program, the largest and most successful fusion device in the world.

The Julian Date (JD)

Is the time that has elapsed since noon January 1, 4713 BC (according to the proleptic Julian calendar), expressed in days and fractions of a day. Julian date was proposed by Joseph Scaliger in 1583.

Johnson Noise (Thermal Noise)

The noise generated by thermal agitation of electrons in a conductor.

Jodrell Bank Observatory

The Lovell Telescope has stood proudly over the Cheshire Plain for over 40 years. It is the flagship of the Jodrell Bank Observatory which is part of the Department of Physics and Astronomy of the University of Manchester. Setup by Bernard Lovell to study cosmic rays just after the war in 1945.