From the mists of time ...
Those who now have Grey hair will remember these if they had ever looked inside a telephone handset .... actually it wasn't just in the telephone system that this type of microphone could be found. I remember having some antiquities used by the armed services back in the 1930's ~ 1940's and they looked something like this ...
While looking around for that photograph, I saw the funniest thing relating to military use of carbon microphones ... sometimes you need to take a step back from life and have a chuckle
Take a look at the last photograph with this advertisement, talk about 'Mouse Ears' and I wouldn't want to be that signal corps soldier on listening duty, a prime target for the enemy (if they could stop laughing)
I am working through another box of my 'telephone bits & pieces', along with ALL the other stuff that needs to find new, caring folk who are interested in "how things used to be"
Marked on the side of the capsule as A.P.O. although I don't think this means Australian Post Office as the common reference to the Australian telephone communications provider was PMG (Postmaster Generals Department) In the USA it could mean Army Post Office, or locally perhaps Assaulting A Police Officer or even Autopilot Off! Hmmmm ... none of these seem feasible (this is definitely a locally used product and certainly not from USA)
I do remember the "Type 13B" but don't recall the CB after this, perhaps it just means Central Battery.
These carbon mic's were cheap to produce and worked well for a time, but began giving problems after hours of having people spit on the granules when speaking!
Remember the old remedy? If the person you are speaking to cannot hear you clearly or you are hearing 'sizzling' noises in the sidetone, give the handset a good shake! Shake shake shake and the microphone works again ....
Try that with a mobile phone, you will just get strange looks!
A.P.O. - Telephone Instrument Handset SPARE PART
Carbon Transmitter
Type: 13B CB
Green topside paint. I seem to remember the colour mean't something but I can't remember what!
DIMENSIONS:
58.2mm diameter x 7.2mm thick
Central barrel (where the carbon granules 'live') is 20.6mm diameter x 10mm length
Screw on terminals (nuts supplied)
TESTING:
My testing is purely DC, I did not go to the extent of making a simple microphone circuit to suit but based on the test results, I am satisfied the microphone is functioning fine.
Theory of operation is that these microphones are like a voice activated variable resistor. Air movement (the speaker talking or spitting at the microphone) causes the carbon granules to move around in sympathy. This means the resistance across the two terminals keeps changing in the same way.
I have at the moment two capsules, both have been tested.
Lying on the benchtop, after having a little shake to even up the spread of granules, I measured around 2K ohm. Speaking near (these are actually quite sensitive) the mouthpiece and the resistance goes higher in sympathy with the talking. Once talking stopped, the resistance again rested at around 2K
Based on this result, I am confident the microphone(s) capsule are still functioning after all these years.
COSMETIC CONDITION:
USED
CLEAN
NO damage to the metalwork
NO damage to the terminals
One capsule has some light oxidisation to the rear, this is no big deal and does not affect the operation of that capsule.
I have now packed the capsule(s) into clear sealed plastic packets.
NOTE: Although the photograph(s) show two transmitters, the advertisement price is for ONE only, not both
