- Focus on 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. [more]
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
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.
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