which a high negative voltage is ap-
plied to a metal tip placed in an evac-
uated vessel some distance from a
glass screen with aÛuorescent coat-
ing. The tip produces electrons by
Üeld emission, i.e. the emission of
electrons from an unheated sharp
metal part as a result of a high elec-
tricÜeld. The emitted electrons form
an enlarged pattern on theÛuores-
cent screen, related to the individual
exposed planes of atoms. As the reso-
lution of the instrument is limited by
the vibrations of the metal atoms, it
is helpful to cool the tip in liquid he-
lium. Although the individual atoms
forming the point are not displayed,
individual adsorbed atoms of other
substances can be, and their activity
is observable.
Üeld-ionization microscope
(Üeld-ion microscope; FIM)A type of
electron microscope that is similar in
principle to the *Üeld-emission
microscope, except that a high posi-
tive voltage is applied to the metal
tip, which is surrounded by low-
pressure gas (usually helium) rather
than a vacuum. The image is formed
in this case by Üeld ionization: ion-
ization at the surface of an unheated
solid as a result of a strong electric
Üeld creating positive ions by elec-
tron transfer from surrounding
atoms or molecules. The image is
formed by ions striking theÛuores-
cent screen. Individual atoms on the
surface of the tip can be resolved
and, in certain cases, adsorbed atoms
may be detected. See also atom-probe
field-ion microscopy.
ÜllerA solid inert material added to
a synthetic resin or rubber, either to
change its physical properties or sim-
ply to dilute it for economy.
Ülm badgeA lapel badge contain-
ing masked photographicÜlm worn
by personnel who could be exposed
to ionizing radiation. TheÜlm is de-
veloped to indicate the extent that
the wearer has been exposed to
harmful radiation.ÜlterA device for separating solid
particles from a liquid or gas. The
simplest laboratoryÜlter for liquids
is a funnel in which a cone of paper
(Ülter paper) is placed. Special con-
tainers with a porous base of sintered
glass are also used. See also gooch
crucible.Ülter pumpA simple laboratory
vacuum pump in which air is re-
moved from a system by a jet of
water forced through a narrow noz-
zle. The lowest pressure possible is
the vapour pressure of water.
ÜltrateThe clear liquid obtained by
Ültration.
ÜltrationThe process of separating
solid particles using aÜlter. In vac-
uumÜltration, the liquid is drawn
through theÜlter by a vacuum
pump. UltraÜltrationisÜltration
under pressure.FIMSee field-ionization micro-
scope.Üne chemicals Chemicals pro-
duced industrially in relatively small
quantities and with a high purity;
e.g. dyes and drugs.Üneness of goldA measure of the
purity of a gold alloy, deÜned as the
parts of gold in 1000 parts of the
alloy by mass. Gold with aÜneness of
750 contains 75% gold, i.e. 18 *carat
gold.Üne structure Closely spaced spec-
tral lines arising from transitions be-
tween energy levels that are split by
the vibrational or rotational motion
of a molecule or by electron spin.
They are visible only at high resolu-
tion. HyperÜne structure, visible only
at very high resolution, results from
the inÛuence of the atomic nucleus225 fine structure
f