Down the pub tonight (so it may not all make sense:-)
A colleague said that during the fifties he and his friends used to make a simple messaging system from a pair of earphones connect together by a simple twisted pair - no batteries - and that he could talk to his friends over quire a distance.
So the questions are:
would this work?
what would the maximum distance be?
In the 50's, we used a couple of tin cans connected by string, and that worked, so an earphone version would have been a luxury ;-)
IIRC (and it's a long shot) you used to be able to get galenium earpieces, and I think they would generate a small electrical current when "activated" by sound - and would recreate the same sound at the other end.
Not a clue about the range though :-}
A quick google shows "crystal earphone" returning quite a few usable hits, although fashion seems to have hijacked the "crystal" bit, so you find daft swarovski tat amongst the results :-}
Alternatively, "piezoelectric earphone"
ISTR that carbon granule transducers were used in the very early days, without power.
"Colin Wilson" wrote in message news: snipped-for-privacy@news.motzarella.org...
It certainly doesn't work with 32 ohm electrodynamic earphones - I just tried it. Although they work quite nicely as a microphone when terminated by a high impedance (approx 150mV peak output when spoken into in a normal voice), this figure drops to almost nothing when you load one earpiece with another of similar impedance. I could see it working ok with crystal earpieces though, as these will produce upwards of half a volt with a decent sound level, and a similar high impedance earpiece connected via a length of cable, should easily respond to produce sound, without unduly loading the sender earpiece. I would think that the limiting factor as to distance, given that this is a very high impedance system, will be cable capacitance rather than resistance, so may actually be rather less than you might imagine. I would have thought though that you could probably get a couple of hundred feet ...
Arfa
I seem to remember playing around with army surplus earphones in the early
50s, and seeing articles in comics or maybe even a very old WW or Popular Electronics or similar suggesting that they could be used as an unpowered intercom. Don't ever remember getting anything to work, though......
I did the same - no power. They were moving iron earphones. Clearly the moving diaphragm created a small voltage.
We used t'dream of tins cans...
I thought they categorically needed a battery. Certainly the instructions I used to try to do that said so.
Yes, it works with a pair of crystal earpieces (which also work as self-powered microphones). I remember finding this out somewhere around age 11 when making a crystal radio, about the same age today's kids start their families.
Distance would probably be limited by cable capacitance, given the high impedance and low signal level of the crystal microphone/earpiece, but with something like an old phone line with two separated bare conductors strung on insulators, it would probably go a long way.
Indeed. They're just a variable resistance.
I think that sound powered phones were used during the first world war. They did however have a hand crank for generating a ringing voltage.
SteveW
The original telephone mouthpieces employed a carbon granule mic. It is a totally passive device, and works by varying the resistance in a current path which includes an electrodynamic earpiece at the remote end. It does this by employing broadly spherical granules of carbon in loose contact with one another, trapped inside a chamber with conductive side plates at opposite ends. The diaphragm which collects the sound waves, is physically connected to one side. The sound vibrations that this picks up, cause the wall of the chamber to flex minutely in sympathy, which either slightly compresses the granules, making each one have a greater area of contact with its immediate neighbours, thereby reducing the resistance path, or the opposite when it flexes back the other way.
An external current source such as a battery, is then definitely needed to make use of this varying resistance to produce an electrical signal for transmission.
Arfa
These would have been moving Iron headphones - consisting of a pair of solenoids and an iron diaphragm. The distance they worked over depended on the connecting cable resistance and the headphone impedance. I used a pair between my bedroom and a friends house a few doors away until the neighbours complained about the cables across thier gardens!
A pair of lodspeakers will produce some sound when used in this way, but not at a high level.
Harry
>
They were the microphone in phones - but the receiver was moving magnet. And I don't think a carbon granule type actually generates a signal - it just changes resistance with movement. A moving coil device - like most headphones - does.
I wonder if an electret condenser mic connected via twisted pair to a standard earphone would work without power..
Yes in 1955 I had a no-battery telephone system in my classroom using some rather unusual earphones. Perhaps they had a moving magnet and coils. I didn't try it for more than a few metres. I removed the dirt from between the floorboards, laid the wire there and put the dirt back so the wires could not be seen.
I remember these "surplus" earphones too (sighs and thinks of Lisle Street and the shop on Church Road, SE19, where I used to drool over HROs and something with a name like 1155?)
The earphones had ferromagnetic* diaphragms held over a bakelite drum which had two enamelled copper coils wound around a piece of iron strip, bent into a U shape. There was a gap of about a millimetre between the poles of the armature and the diaphragm. The varying current in the coils would accelerate the diaphragm relative to the armature.
I used mine with a crystal set and later with a superhat radio with quench control and a mains energised loudspeker that I switched out for under the bedclothes listening to Radio Luxemburg.
Although irrelevant to their use as earphones, I'd guess that the armature would have become permanantly magnetised, so that it would act in reverse as microphone.
You can use modern moving coil headphones as microphones, I've not tried connecting two pairs together, but if you plug one into a preamplifier Aux input, it makes a crude pair of microphones. (Earphones, were what we'd now call headphones. My how those bakelite earcups made your ears sore after you'd worn them for a few hours!)
What are 'crystal' earphones. Piezoelectric? When did they become available?
*They were painted black and had a tendency to rust.
Oh, you mean *those* parts of Lisle Street...
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