A database system which does not have strict layout constraints as regards record formats,

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## F ROTHER

A substance used to promote the formation of a foam in the froth flotation process,

## F STATE

That of an orbital electron when the orbit has angular momentum of three Dirac units.

## F UMAROLES

Small vents on the flanks of a volcanic cone, or in the crater itself, from which gaseous products eman

## F-CENTRE

An electron trapped at a negative ion vacancy in an ionic crystal, /--centres can be formed by the release of electrons by irradiation with X-rays or by producing stoichiometric excess of anions in the crystal, and give rise to broad optical absorption bands. E-centres consist of F- centres with a further electron trapped in the same vacancy. fA-centres are /--centres modified by one neighbouring cation being different from the cations of the lattice, /--centre aggregates can be formed by arrays of nearest neighbour centres.

## F-CLASS INSULATION

A class of insulating material which will withstand a temperature of 155 C.

## F-DIAGRAM

The cumulative residence time distribution in a continuous flow system, plotted on dimensionless co-ordinates. Important in assessing the performance of chemical reactors, kilns etc.

## F-DISPLAY

Type of radar display, used with directional antenna, in which the target appears as a bright spot which is off-centre when the aim is incorrect.

## F-DISTRIBUTION

The distribution of the random variable formed by taking the ratio of two chi-squared random variables, each of which is divided by its degrees of freedom. The central F-distribution is indexed by two parameters, the degrees of freedom associated with the chi- squared variables which form the numerator and denominator of the ratio.