Evening star., 1925-02-21
ACTION OF CRYSTAL
DETECT0R, T0P|C|
Writer'S Idea of a "Cat
Whisker's" Operation in
Catching Radio Signal.
ny DIR. f.. K. FREE.

In an interesting series OF papers
on the theory and practice of the
crystal detector published in a Lon
don periodical. James Strachan sets
forth a, new idea Of what 1t Is that
makes a crystal and cat whisker
operate to detect a radio signal
The essential action of a crystal de-
tector consists. it will be remembered,
in q rectitication of the electric cur
rent. OF the radio-frequency oscilla-
tions that surge from both directions
against the contact of the crystal and
the cat whisker only those moving in
one direction get through. The re-
verse pulses are stopped. The crystal
is essentially, a one-way valve for
the electric current.
DisclOses New Theory.
The new theory assumes that this
rectifying action takes place alto
gether in the surface layers Of the
crystal. The molecules of galena,
for example, are assumed to be
little less rigidly fixed in this surface
layer than in the mass of the crystal
These surface molecules can move
around n little; can turn sometimes
on their axes as cork on the sur
face of a river will turn over and
over as it floats down stream. 1t is
assumed, Furthermore, that the cleo
tEic properties of galena molecule
are not the same in all directions.
1t follows that IL all the molecules
in the surface layers are turned, say,
on their ends, the conductivity across
the surface will be different than
when all the molecules are turned on
their sides. Furthermore, If all the
molecules are on end the conductivity
may be greater in one direction than
in tho other, for the molecules may
be more permeable to the current In
this direction than in the reverse one.
The molecules, in other words, may
themselves be tiny rectifiers.
Mystery Begets Mystery.
ITO the casual observer it may seem
that this suggestion merely substi-
tutes tweedledum for tweedledee. For
the mystery of why a crystal rectifies
it merely substitutes the other mystery
of why a molecule rectifies. But
this is not quite a fair criticism. Mr.
Strachan's papers-which contain, by
the way, many useful facts about
crystal detectors-will have perform
ed a real service If they focus the at
tention of crystal students on the i'm
portancc of the surface layers, pos
sibly only three or four atoms deep,
with which the cat Whisker actually
makes it contact
1t is somewhat in this surface layer
Jndt we Will 1lnu the secret Ol
action. To the editor of this depart
ment it seems probable that the most
important part of this surface layer
is the film of absorbed gas
On any solid surface. especially on
the surface of any metal, there is al
ways IA tightly attached layer of
atoms of gas, mostly the oxygen and
nitrogen gases of the air. 1t is this
attached gas--called the absorbed"
gas-so difficult to detach during
evacuation from the tungsten parts Of
tubes.
Layers OF Absorbed Gas Cited.
There is one of these layers of alb
sorbed gas two or three atoms thick
on the surface of every crystal and
on every cat whsker point. The exact
nature of the absorbed layer will de-
pend on the chemical composition of
the crystal, on the nature of it's crys-
tai surface just at that point and on
its past history-for example, on its
cleanliness.
Nobody knows how this layer of
stuck fast gas atoms will affect cleo
trio conductivity across the gap. The
matter should be studied by physicists
and by radio engineers.
(Copyright. 1923, Popular Radio Incl