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Hacker, Robert W. (ed.) / The Wisconsin engineer
Volume 53, Number 4 (January 1949)

Traeder, Howard
Science highlights,   p. 12


Page 12


Science Highlights
                         by Howard Traeder m'48
ELECTRON DIFFRACTION
INSTRUMENT
  This new General Electric device
utilizes electrons to study a surface
layer of metal less than a quarter
millionth of an inch thick. The in-
strument either shoots a beam of
electrons through the extremely thin
sheet of metal, or it diverts the beam
at an angle to the surface of the
metal. This enables engineers to de-
termine a variety of surface condi-
tions, including corrosion and crys-
tal structure of the molecules.
  During operation of the instru-
ment, a beam of electrons is "boiled
off" a white-hot tungsten filament
and focused by means of a magnetic
"lens" in much the same manner as
a beam of light is focused with a
glass lens. The chamber in which
the electrons travel is evacuated to
a high degree, since electrons would
be quickly dispersed by molecules
of air or dust. The pressure in-
side this chamber is approximately
1/8,000,000 the pressure of the at-
mosphere.
  In one application, the electron
beam is passed through a two-mil-
lionths-inch-thick section of metal
and produces an image on a fluore-
scent screen, or on a strip of ordi-
nary photographic film if a perma-
nent record is desired.
  In other applications, the beam,
directed at an angle to the metal
surface, passes through tiny surface
projections which bend the beam
and direct it at a screen or film. The
image produced is a series of con-
centric circles, which differs for
each type of crystal structure. Engi-
neers can readily determine from
the photographs both the nature of
the corrosion on the surface and the
crystal arrangement of the mole-
cules of metal.
CERAMIC-METAL BOND
  Radio tubes of ceramic materials,
recently developed by General Elec-
tric scientists, offer many advantages
in producing tiny radio waves a few
inches in length. Obtaining a tight
ceramic to metal seal was the chief
problem in making such tubes. In
the new technique, the materials are
joined by an alloy of silver and tita-
nium. This is so strong that if the
bonded piece is broken, the break
occurs in the ceramic and not in
the bond. The method may also be
used for joining two pieces of cera-
mic together.
  The soldering process is done in
a vacuum at high temperatures
which drive out gases in the metal
and ceramic parts, thus simplifying
final evacuation of the tube.
RCA DEVELOPS NEW
FINE MESH SCREEN
  The Radio Corporation of Ameri-
ca was recently confronted with the
task of producing a copper screen
with 250,000 openings to the square
inch in conjunction with the devel-
opment of the sensitive-image orthi-
con television camera tube now in
wide use in the nation's television
studios. Since the electron image of
the scene to be televised is focused
on this screen, the mesh must be ex-
tremely fine to prevent its being
visible in the picture when viewed
at the receiver.
  The finest mesh screens available
before the war were woven wire
screens, or electrolytic screens, with
about 400 holes per linear inch, or
160,000 openings to the square inch.
These screens were non-uniform in
the arrangement of openings, how-
ever, and passed less than 40'/c of
the electron image. For a transmis-
sion of 50( , the best screen then
available was made of woven wire
with only about 200 mesh. It was
soon realized that the non-uniformi-
                         (Cit cotirtesy General Electric Co.)
The new GE electron diffraction instrument.
ties and relatively coarse
      (pletae turn to page 24)
THE WISCONSIN ENGINEER
mesh
12


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