Did you find out why the type 21 failed?


Yes, whilst the 13 had used solid carbon electrodes, the 21 used
"carbonised" steel electrodes. Due to surface contamination on the metal
prior to carbonisation there was not always a satisfactory bond at the
molecular level between the steel and the carbon, this allowed the surface
of the steel to corrode under the carbon layer. The solution was to improve
the cleaning of the surface of the steel prior to carbonisation.

Interestingly there was, even in the 1960s by which time when carbon
microphones had been in common use for some 80 years, no really satisfactory
theory of how they worked. The common theory at the time was that the
contact area between adjacent granules increased with increasing pressure,
but this could not explain why the *average* resistance increased with
increasing sound level. The theory that the group I was working in arrived
at was that the granules bounced off each other whilst activated by sound,
so that each point of contact became a sort of pulse-width modulator.

David.

The Panasonic GX series office telephone system which was in production in the 1990s
used carbon mics in the system telephones.


--
Graham.

%Profound_observation%

In article , David Looser
wrote:


Interestingly there was, even in the 1960s by which time when carbon
microphones had been in common use for some 80 years, no really
satisfactory theory of how they worked. The common theory at the time
was that the contact area between adjacent granules increased with
increasing pressure, but this could not explain why the *average*
resistance increased with increasing sound level.

I've just been reading a recent 'Physics World' magazine. This has an
article on 'topological insulators'. These are an example of a material
which has different properties for the bulk and for the surfaces or edges.
All kinds of odd properties can arise as a result. Wonder if the carbon
was/is doing something like this. The more people study the behaviour of
various carbon structures (tubes, graphine, etc) the more of a range of
behaviours they find!

Slainte,

Jim

--
Please use the address on the audiomisc page if you wish to email me.
Electronics http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Armstrong Audio http://www.audiomisc.co.uk/Armstrong/armstrong.html
Audio Misc http://www.audiomisc.co.uk/index.html

"Dave Plowman (News)" wrote in
message

In article ,
Iain Churches wrote:

Carbon mics were not used in recording, but they were
described briefly in text books. The explanation you
write above is the one I remember reading.

I'll bet they were in the early days. The BBC certainly
used them for a while.

Carbon mics were exclusively used in telephones in the US up until the 1960s
or so. They were replaced by a dynamic element and a transistor amplifier.

Carbon mics were used widely by the miliary through WW2.

The competitive/alternative technology involved ceramic and cystal
piezoelectric elements.

"Graham." wrote in message
...


The Panasonic GX series office telephone system which was in production in
the 1990s
used carbon mics in the system telephones.


Suprisingly late. Most new telephone types introduced during the 1980s used
moving-coil or electret microphones. Single-chip telephone transmission
chips were readily available by then which included send and receive
amplifiers, loop-current controlled gain control and an electronic hybrid.
BT started using an elecret microphone with integrated send amplifier as a
"drop-in" replacement for the carbon insert 21 in the 700 series telephones
during the '80s. It was exactly the same size and shape, and with the same
terminals, the only obvious visable difference was the colour of the plastic
body.

David.

David Looser wrote:

"Iain Churches" wrote

Did you find out why the type 21 failed?


Yes, whilst the 13 had used solid carbon electrodes, the 21 used
"carbonised" steel electrodes. Due to surface contamination on the
metal prior to carbonisation there was not always a satisfactory bond
at the molecular level between the steel and the carbon, this allowed
the surface of the steel to corrode under the carbon layer. The
solution was to improve the cleaning of the surface of the steel
prior to carbonisation.

Interestingly there was, even in the 1960s by which time when carbon
microphones had been in common use for some 80 years, no really
satisfactory theory of how they worked. The common theory at the time
was that the contact area between adjacent granules increased with
increasing pressure, but this could not explain why the average
resistance increased with increasing sound level. The theory that the
group I was working in arrived at was that the granules bounced off
each other whilst activated by sound, so that each point of contact
became a sort of pulse-width modulator.

David.

In the late 50s I made my own carbon mic. It consists of two carbon
rods (from U2 batteries) mounted in parallel about 1" apart on a small
cardboard box (to act as a sounder). A length of pencil lead was rested
across the rods in grooves filed in the rods. It worked after a fashion
.

--
Ashley
For Windsor Weather see www.snglinks.com/wx

On 2 Mar,
"Ashley Booth" wrote:

In the late 50s I made my own carbon mic. It consists of two carbon
rods (from U2 batteries) mounted in parallel about 1" apart on a small
cardboard box (to act as a sounder). A length of pencil lead was rested
across the rods in grooves filed in the rods. It worked after a fashion

Mine (of the same vintage) had the two U2 rods at right angles perched on a
cigar box. I can't remember the details. It also worked after a fashion.
Probably like my ears now.

--
BD
Change lycos to yahoo to reply

"Ashley Booth" wrote in message ...
David Looser wrote:

"Iain Churches" wrote

Did you find out why the type 21 failed?


Yes, whilst the 13 had used solid carbon electrodes, the 21 used
"carbonised" steel electrodes. Due to surface contamination on the
metal prior to carbonisation there was not always a satisfactory bond
at the molecular level between the steel and the carbon, this allowed
the surface of the steel to corrode under the carbon layer. The
solution was to improve the cleaning of the surface of the steel
prior to carbonisation.

Interestingly there was, even in the 1960s by which time when carbon
microphones had been in common use for some 80 years, no really
satisfactory theory of how they worked. The common theory at the time
was that the contact area between adjacent granules increased with
increasing pressure, but this could not explain why the average
resistance increased with increasing sound level. The theory that the
group I was working in arrived at was that the granules bounced off
each other whilst activated by sound, so that each point of contact
became a sort of pulse-width modulator.

David.

In the late 50s I made my own carbon mic. It consists of two carbon
rods (from U2 batteries) mounted in parallel about 1" apart on a small
cardboard box (to act as a sounder). A length of pencil lead was rested
across the rods in grooves filed in the rods. It worked after a fashion
.

I vaguely remember making one in the mid '60s I seem to recall I used a small stump
of a carbon rod for one electrode and the rest of it I pulverised for the granules
I cant remember a second carbon rod, I thing the capsule might have been metallic.
I've no memory of the diaphragm, nor any of the finished mic in use which leads
me to believe the whole enterprise was a failure.

Our school text books showed the carbon rod in a dry cell surrounded by a muslin
bag containing the MnO2 depolariser, but I never found such a bag in all the U2s I
dissected. Was it present in the era you are talking about? Perhaps it goes further back
than that and my school's text books and teaching practices weren't renowned for
being modern.


--
Graham.

%Profound_observation%

wrote in message ...
On 2 Mar,
"Ashley Booth" wrote:

In the late 50s I made my own carbon mic. It consists of two carbon
rods (from U2 batteries) mounted in parallel about 1" apart on a small
cardboard box (to act as a sounder). A length of pencil lead was rested
across the rods in grooves filed in the rods. It worked after a fashion

Mine (of the same vintage) had the two U2 rods at right angles perched on a
cigar box. I can't remember the details. It also worked after a fashion.
Probably like my ears now.

Did you also make a crystal set san crystal, instead using a razor blade
and pencil lead? I thought it was pretty novel at the time but a Google such suggests
it was a well known arrangement.

--
Graham.

%Profound_observation%

Graham. wrote:

Did you also make a crystal set san crystal, instead using a razor blade
and pencil lead? I thought it was pretty novel at the time but a Google such suggests
it was a well known arrangement.


Apparently this was commonly done by soldiers in the Second World War.

Bill

"Dave Plowman (News)" wrote in message
...
In article ,
Iain Churches wrote:
Carbon mics were not used in recording, but they were
described briefly in text books. The explanation you
write above is the one I remember reading.

I'll bet they were in the early days. The BBC certainly used them for a
while.

Interesting, Do you know what did they use them for?

The Schottky ribbon dates from 1923
and ribbon mics were available from the very
earliest days of electrical recording and
ubiquitous by early 1930s

Even by modern standards, ribbons can produce
a very pleasing sound. I have a pair of British made
Film Industries M8s which are excellent on strings,
and the STC/Coles 4xxx series is still highly regarded
by people who have them. Especially pleasing for
acoustic guitar.

Iain