Carbon mics & anthracite coal

[Conrad Hoffman]

I've been messing with microphone "hummers" used to generate a signal in old bridges. They're quite simple, with a small carbon mic picking up the vibrations of a reed or tuning fork, and modulating the current to keep it going. No tubes, transistors or any other parts, save a transformer to get the right output impedance. The mics are capable of modulating tens of mA, which seems quite remarkable.

Same technology as old telephones that used carbon mics. Reading various things on the 'net, it seems the carbon is anthracite coal powder that's been graded and heat treated, probably to drive off the remaining volatiles. I want to know more specifics- does anybody know the mesh size for the powder, the exact treating process or any other useful info. It seems easy enough to get a chunk of anthracite and DIY your own mic. Also, they are said to slowly wear out. What's the mechanism and the cure? Just shake it up, or does the powder need to be replaced?

[Tom45]

That brings back some memories.

Once in my youth I connected a carbon microphone from a telephone in series with a battery and a speaker.  When  turned on with the microphone next to the speaker, it squealed with feedback! Not an active element anywhere. Surprised the heck out of me.

As for the details, my memory from >60 years ago is that the carbon granules were fairly coarse. Certainly not a powder.

Those carbon microphones from telephones were once ubiquitous. Probably have to go to eBay to find one now.

[David Hess]

Once in my youth I connected a carbon microphone from a telephone in series with a battery and a speaker.  When  turned on with the microphone next to the speaker, it squealed with feedback! Not an active element anywhere. Surprised the heck out of me.

Carbon microphone elements have gain.

[chris_leyson]

From a long time ago I also remember them being filled with carbon granules rather than power. Even did the battery and speaker thing, amazing. These seemed to be in general use in the 60's http://www.britishtelephones.com/trans16.htm

[james_s]

I remember playing with those carbon mic elements. Yet another example of something that was everywhere and then suddenly gone.

[T3sl4co1l]

Heat treatment makes coke.  It may be amorphous or graphitic in nature, plus some ash content.  If not fully heated, some volatile content will be left as well (probably aromatic in nature, heavy tar sorts of things, would probably result in lower conductivity).

Would imagine granules of activated charcoal or graphite would be a good start, maybe a blend even.

Wood charcoal is probably more porous than coke (which itself is still porous), so would tend to crumble over time; that's probably the wear mechanism.

Very fine powders are unsuitable because they'll become airborne, and the resistance won't modulate right.  At least, not at the right rate, you'd get a weird time constant following large excursions or something.  This implies the grain size should correspond to the desired frequency response.

Other moderately conductive substances may do.  Black sands (magnetite), maybe?  Or regular sand with some metal powder or flake blended in?

Tim

[Gyro]

I too remember playing with and dismantling carbon microphones as a kid. As I remember, the granules where hard and 'shiny', much like the size and consistency of granulated sugar, certainly no powder or dust. On GPO telephone microphones, the center electrode was graphite faced.  I once took a Tannoy tank microphone apart, that had a ring of about a dozen cells. the connections to the diaphragm being wire rods driving metal plates suspended in the middle of the carbon filled cells , presumably some form of noise cancelling.

Irrc, the main enemy of carbon granule microphones was moisture, which would quickly cause failure. Telephone handsets did respond well to the occasional shake or knock. Presumably, like any irregularly shaped granules the will slowly settle and interlock causing more rigid higher pressure contact points between the granules which would probably reduce sensitivity. It's hard to think of any other 'wear-out' mechanism.

There are a few ebay UK cheap listings for GPO phone microphones, presumably you could find the same in the US. The degree of moisture sealing is clear to see...

https://www.ebay.co.uk/sch/m.html?_odkw=&_ssn=loz543&hash=item25fa7ed008%3Ag%3AhVkAAOSwyf1bMTOP&item=163116404744&_osacat=0&_from=R40&_trksid=p2046732.m570.l1311.R1.TR1.TRC0.A0.H0.Xcarbon+microph.TRS0&_nkw=carbon+microphone&_sacat=0

[LaserSteve]

 Try BSTJ, The Bell System Technical Journal and similar publications from AT&T. Should have the complete processing method in the article.

S.

[LaserSteve]

 Looks like you need some Cabosil too:

US patent 3859478A has some details.
So does US3835266A, work backwards from those two.

The second patent describes mesophase pitch, I spent years in a lab making carbon fiber from it.

Steve

S.

[Conrad Hoffman]

Great stuff, thanks! It sounds like we all grew up playing with the same stuff. Very useful on the patents and it gave me clues on the search terms to find more.

[Tom45]

Carbon microphone elements have gain.

Yes they do.

Up until that experiment I thought gain required >= triode, or a transistor. Hence the surprise.

Now I know that at least tunnel diodes and the lowly carbon microphone can be added to the list.

[Doctorandus_P]

A similar technology is used in speed regulators for sowing machines.
I once destructed an old sowing machine, with an around 15W 230Vac motor.
The foot pedal had a ceramic tube in it (15 to 20mm diameter, 5 to 10cm long) and in that tube was a stack of carbon disks, and that was in series with the motor.

Speed was regulated by putting a (very light) pressure on the stack of carbon disks to change it's resistance.

[Gyro]

There were (are?) carbon pile regulators too...

https://www.brighthubengineering.com/commercial-electrical-applications/62223-what-is-a-carbon-pile-regulator/

There are even a bunch on ebay:

https://www.ebay.com/sch/i.html?LH_CAds=&_ex_kw=&_fpos=&_fspt=1&_mPrRngCbx=1&_nkw=carbon+pile+voltage+regulator&_sacat=&_sadis=&_sop=12&_udhi=&_udlo=&rmvSB=true&_fosrp=1

[Doctorandus_P]

Another old element with gain is a special kind of inductor with 2 windings.

The idea is that you put a small DC current through a winding with a lot of loops, and this brings the core of the inductor into saturation. If the core is saturated the inductance of the other winding dops and it will pass a big AC current. This was used to control multiple kW electric motors in the old days

[David Hess]

Another old element with gain is a special kind of inductor with 2 windings.

The idea is that you put a small DC current through a winding with a lot of loops, and this brings the core of the inductor into saturation. If the core is saturated the inductance of the other winding dops and it will pass a big AC current. This was used to control multiple kW electric motors in the old days

Magnetic amplifier regulation in off-line ATX switching power supplies is common.  The +12 volt output is directly regulated through the primary side and the +5 and +3.3 volt outputs have secondary side magnetic amplifier regulation.

[Gyro]

Ah yes. Saturable Reactors, also referred to as magnetic amplifiers. Now they are still used.  http://magspecinc.com/reactors/saturable-reactors/

Including BIG ones! http://www.powermagneticsinc.com/products/reactors/saturable-reactor/

... but we digress.

[chris_leyson]

Never had much sucess with saturable regulators, control loop stability was the problem, not the regulator. Used Ferroxcube 3R1 toroids and it took ages to wind the control windings.

[SeanB]

Funny enough I used to repair worn out carbon microphones, using an electret capsule, a resistor and a PNP transistor to get the required current gain for the system. In the intercom you had a 600R speaker ( ok a transistor amplifier that emulated a 600R speaker, using a 3 transistor amplifier and driving a 8R speaker in the outdoor unit) and a carbon microphone, and they would wear out, with replacement being expensive.  Fitted the electret capsule in the space occupied by the carbon granules, and put the transistor and resistor in there as well, then placed the front cover back on, so the microphone was again a drop in unit on the phone, fitting the same space and using the same 2 screw connections for the wire. They were a lot louder than the carbon mics as well, a bonus in a street situation as the inside did not have to shout.

[james_s]

What wears out in carbon microphones? Do the granules break down? Absorb moisture?

[Zero999]

What wears out in carbon microphones? Do the granules break down? Absorb moisture?

I think the consensus is moisture, as mentioned above.

Carbon microphone elements have gain.

Yes they do.

Up until that experiment I thought gain required >= triode, or a transistor. Hence the surprise.

Now I know that at least tunnel diodes and the lowly carbon microphone can be added to the list.

Carbon microphones coupled directly to speakers, were used as repeaters, before thermionic valves were invented. They were quite noisy and suffered from high distortion, so weren't much good.

I have experimented with a carbon mic from my parents' old phone, but I never got as far as dismantling it. I wasn't surprised that it had gain, but I didn't know that most microphones don't have gain, back then.

[cncjerry]

For the longest time I had remembered playing with a broken phone, we had made a one way intercom, and was thinking we had only the mic, speaker and a battery.  I often tried to remember how that worked with just a few parts, no actives,  and it became a puzzle for me. Thanks for clearing this up. 

Jerry