Evening star., 1926-10-17
No. 86: Clear a. a Crystal.


Smith had been trying a small
crystal set to determine what advan-
tages, il any. it offered over tubes.
when he reached conclusion regard
ing clarity. Il will admit this" he
said. its certainly clear
"But do you know why?" I asked.
"No."
Then" added. ut isn't 80 clear
after ALL"
few moments later we were deep
into the mysteries Of crystal detec-
tors. Smith never before had realized
how puzzling such simple thing
could be.
II don't see how crystal like this
can alter the nature Of electric cur
rent passing through it," he re-
marked.
Neither does any one else" ad-
mitted. There are lot of good
guesses. however, and il helps you
with radio in general to know what
these are.
I'm the first place, YOU should
know that there are two kinds of
crystals. One is the adjustable type
requiring a cat whisker; the other
is the fixed crystal. Both are highly
complicated in so far as the theory of
their operation is concerned. but
amazingly simple in their actual
operation. Yours Is the cat whisker
type, so perhaps we had better con-
centrate on that, leaving the other
for later date.
1.-The fact that you have to fool
around with the cat whisker until
you find point where the fine wire
meeting a point on the crystal brings
in reception hest makes 1t plain you
do MT have to he very observant to
appreciate that the crystal 10 not uni
form in it's detective properties. Such
crystals are said to possess property
known as unilateral conductivity.
Current can flow through in one di-
rection better than in another. Fur
thermore. the wire in contact with
the crystal has much to do with the
process OF hanging the current from
the two-way to the one.way kind
necessary for operation of the
phones.
Crystals Rectify.
"The CRYSTALS Of the second class
rectify because they do not have high
regard for Ohm's law. Variations In
applied voltages cause several Oddi-
ties in action. with the result that
while ,such crystals change the In
coming current from radio frequency
70 audio they do 1t in quite a diner.
ent way And before I forget 1t l
should tell you that there are crystals
that fall somewhat in both groups.
"Crystals are usually oxides and
sulphides. The one most frequently
used for the cat whisker' business 1s
lead sulphide and is popularity known
fn1
tie and carhorundum are all possible
AZ CRYSTALS for detection. Silicon and
ca arb6rundum are used for the second
group where the contact is under
enough pressure to call the adjust
ment fixed.
Now let's see II I can give you
picture of the way the cat whisker'
combined with galena works. II you
examine such a crystal. you Will see
that it Is covered with fine serratiOns
and that they run In one direction
only. You have noticed, perhaps. that
YOU get results only when you touch
the 'whisker' against the saw-tooth
edges or perhaps in one OF the de-
pressions on the crystal.
III 1t is so difficult to find q point
on crystal where 1t will work. you
can readily appreciate that only
small cross.section of the crystal Is
being used to conduct current when
correct adjustment is made. This
Is important. Il has much to do with
the capacity of the crystal to change
one kind Of current into another. The
theory is that because this current at
radio frequency is confined to such a
small area of the crystal. the energy
heins picked up by the aerial and
ground is wasted in the form OF
heat at the point where the cat