A microscope that uses a beam of electrons
rather than light to study objects too small
for conventional microscopes. First constructed by Max Knoll and Ernst Ruska
around 1930, the instrument consists typically of an evacuated column of
energetic lenses with a 20–1,000 kV electron gun at the top of a fluorescent
screen or photographic plate at the bottom; it can thus be thought of as
a kind of cathode ray tube. The
various lenses allow the operator to see details almost at the atomic level
(0.3 nm) at up to a million times magnification (though many specimens deteriorate
under the electron bombardment at these limits), and to obtain diffraction
patterns from very small areas.
|Section through a Philips EM400 electron microscope.
(A) Electron gun. (B) Anode. (C) Centering coils. (D) Vacuum lock.
(E) First condenser lens. (F) Second condenser lens. (G) Alignment
coils. (H) Condenser diaphragm. (J) Objective lens. (K) Specimen chamber.
(L) Objective diaphragm. (M) Diffraction lens. (N) Intermediate lens.
(O) Firs projector lens. (P) Second projector lens. (Q) Binoculars.
(R) Vacuum lock. (S) 35mm camera. (T) Focusing screen. (U) Film magazine.
(V) Fluorescent screen.
In the scanning electron microscope, the beam is focused
to a point and scanned over the specimen area while a synchronized television
screen displays the transmitted or scattered electron intensity.
In the scanning tunneling microscope a tiny metal probe
on a movable arm is held in near contact with the specimen, which has to
be a conducting material, and scanned across it. An electrical signal, produced
when electrons tunnel between the specimen and the probe, is kept constant
by raising and lowering the arm. The necessary movements are analyzed by
a computer, which generates a contour map of the surface displayed on the
Electron microscopes are used for structural defect and composition studies
in a wide range of biological and inorganic materials.