From: rick@ee.uwm.edu
Subject: Detector != Diode
Summary: Was: How old is the oldest Diode?
Reply-To: rick@discus.mil.wi.us
Organization: Jus' me.
Date: Mon, 13 Apr 1992 17:52:33 GMT

lrk@k5qwb.lonestar.org (Mr. Lyn R. Kennedy) writes:
>irvine@en.ecn.purdue.edu (B-rent) writes:
>
>> jqtaylor@hpdmd48.boi.hp.com (John Q
>> > 
>> > Didn't some soldiers in the Great War use a piece of pencil lead
>> > fastened to a wire pin pushing down upon a razor blade to detect
>> > radio signals from their foxholes? Does this constitute a diode?

	No.  This does *not* constitute a diode.  Current may flow in
both directions through such a device with equal ease.

>> 	It works under the same principles, but I think they had
>> 	a number of other detectors (galena & cats whisker) that
>> 	worked with a heterojunction.  They weren't called diodes.

	No, it doesn't.  There seems to be a common misconseption that
only a diode can be a detector... this is wrong.

	Some of the earliest detectors worked by means of COHESION.
Remember, back in those days they didn't do much tuning.  Everyone
spark-gapped the *whole* *band*, though attempts to concentrate
the signal near some frequency or other did seem to help things... :-)

	Sometimes, the detector was a small capsule filled with some
very fine filings of (usually) an Antimony alloy.  When sufficient
voltage is applied to such a device, its particles 'cohere' to form
a more-or-less conductive path.  It was used by 'biasing' it very
close to this voltage (DC) and then applying the high ends of the
LC circuit through capacitive coupling to either end of the tube.
Thus, when a radio signal raised the LC circuit's voltage above a
certain point, the 'coherer' would become conductive, driving some
element with the DC current that then passed through it.  Sometimes
the element driven was a sort of glow-plug ignition device... the
first radio-controlled bombs.

	Note that the coherer *remained* conductive, even when the
trigger voltage/current was removed.  To remedy this problem (except
in the case of the bomb...) most receivers of this sort needed some
kind of "tapper".  This was usually an electro-mechanical striker,
just like that in a door-bell (but smaller and gentler) and was
activated by the very current passing through the coherer.  Thus the
voltage caused cohesion, cohesion allowed DC flow, and DC flow caused
the tapper to give the coherer a knock, breaking DC flow and making
the coherer ready for the next voltage build-up across the LC tank.

>> 	So I figure that only the semiconductor junction can be called
>> 	a diode.
>
>Gee, I thought those vacuum tube ones were legitamately called diodes 
>too. :-)

Lyn's right...   The oldest *effective* diode was the ol'
cat's whisker on a hunk of crystal, but the oldest *NAMED* "diode"
was a vacuum tube...  TWO electrODEs, thus the "di-ode tube".

Just so I don't get flamed, there were liquid-dielectric "diodes" too,
but I don't recall any of them being used in radio-freq. applications.
---
Rick Miller  rick@discus.mil.wi.us  discus!rick@uwm.edu  rick@ee.uwm.edu
From: brown@hpspkla.spk.hp.com (Pat R. Brown)
Date: Mon, 6 Apr 1992 20:12:57 GMT
Subject: How old is the oldest Diode?
Organization: Hewlett Packard Company, Spokane, Wa.