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H HRS

The lines of the Balmer series in the hydrogen spectrum. Their wavelengths are: Ha 656-299nm; H/? 486-152 nm: Hy 434-067 nm: Hr 410-194 nm. The series continues into the ultraviolet, where about 20 more lines are observable,

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H-2 HISTOCOMPATIBILITY SYSTEM

The major histocompatibility system in the mouse. H-2 genes determine the major histocompatibility antigens on the surface of somatic cells and also the immune response Ir genes. The antigens in a given strain of mice are controlled by alleles within the H-2 locus. The H-2 system corresponds closely to the HLA system of humans, and much of our understanding of the latter was derived from studies in mice.

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H-CLASS INSULATION

A class of insulating material which can withstand a temperature of 180 C.

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H-DISPLAY

Modified B-display to include angle of elevation. The target appears as two adjacent bright spots and the slope of the line joining these is proportional to the sine of the angle of elevation.

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H-HINGE

A hinge which when opened has the shape of the letter H. Also parliament hinge.

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H-NETWORK

Symmetrical section of circuit, with one shunt branch and four series branches.

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H-PLANE

The plane containing the magnetic vector H in electromagnetic waves, and containing the direction of maximum radiation. The electric vector is normal to it.

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H-RADAR

Navigation system in which an aircraft interrogates two ground stations for distance.

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H-SECTION

An electrical network derived from the T-section, in which half of each series arm is placed in the other leg of the circuit, making the section balanced.

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HA-HA

A fence sunk in a ditch below ground-level so as to give an uninterrupted view,

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