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J CHAIN

A polypeptide chain with a high content of the amino acid cysteine, which enables it to form disulphide bonds. It helps to link together IgA molecules into polymeric forms and to hold IgM in pentameric form.

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J GENE

J exon. A short sequence of DNA coding for part of the hypervariable region of immunoglobulin light or heavy chains near to the site of joining to the constant region. There are several possible J exons of which any may be used. The gene for the beta chain of the T-cell antigen receptor also includes a different J exon which has similar variability,

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J-ANTENNA

Half-wave antenna fed and matched at the end by a quarter-wavelength line.

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J-DISPLAY

A modified A-display with circular time base. See R-display.

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J-J COUPLING

Extreme form of coupling between the orbital electrons of atoms. Electrons showing individual spin-orbital coupling also interact with each other. See intermediate coupling, Russell-Saunders coupling

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JACK

A portable lifting machine for raising heavy weights through a short distance, consisting either of a screw raised by a nut rotated by hand gear and a long lever, or a small hydraulic ram. See hydraulic jack,

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JACK BOX

One containing switches or connections for changing circuits,

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JACK PLANE

A bench plane about 16 in long, used for bringing the work down to approximate size, prior to finishing with a trying or smoothing plane,

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JACK RAFTER

A short rafter connecting a hip-rafter and the eaves, or a valley-rafter and the ridge,

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JACK SHAFT

An intermediate shaft used in loco

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