judith submitted this idea :
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.

In the 50's you could buy war surplus field telephones. They were
un-powered, but used a cranking generator to ring the bell. They used
either a moving coil or moving magnet microphone, which generated the
voltage needed to make the earpiece at the far end produce sound.

So the questions a

would this work?

Yes!
what would the maximum distance be?

They would work over several miles of cable, but they were not very
loud.

--
Regards,
Harry (M1BYT) (L)
http://www.ukradioamateur.co.uk

In article ,
Harry Bloomfield wrote:
In the 50's you could buy war surplus field telephones. They were
un-powered, but used a cranking generator to ring the bell. They used
either a moving coil or moving magnet microphone, which generated the
voltage needed to make the earpiece at the far end p

Think those were called Tele F and had an internal battery.
Old telephones used carbon mics and these don't actually generate a
signal, unlike say a moving coil device.

--
*Experience is something you don't get until just after you need it.*

Dave Plowman London SW
To e-mail, change noise into sound.

In article ,
Harry Bloomfield wrote:
In the 50's you could buy war surplus field telephones. They were
un-powered, but used a cranking generator to ring the bell. They used
either a moving coil or moving magnet microphone, which generated the
voltage needed to make the earpiece at the far end produce sound.

This might provide some more info:-

http://www.btinternet.com/~ALLAN.ISAACS/misc2.html

--
*If you don't pay your exorcist you get repossessed.*

Dave Plowman London SW
To e-mail, change noise into sound.

Jan Wysocki expressed precisely :
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?)

R1155 being the receiver of the 1155/ 1154 pair.

--
Regards,
Harry (M1BYT) (L)
http://www.ukradioamateur.co.uk

On Wed, 18 Feb 2009 00:20:58 +0000, judith
wrote:


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 a

would this work?

what would the maximum distance be?



many thanks to all who responded - particularly for links and photos.

cheers

On 18 Feb 2009 10:51:52 GMT, Jan Wysocki
wrote:



What are 'crystal' earphones. Piezoelectric? When did they
become available?


They were made for the hobbyist / crystal set / 2 transistor radio /
etc market Ca. 1960.

They were Piezoelectric and are still available.

http://www.maplin.co.uk/Module.aspx?ModuleNo=3256&C=Froogle&U=3256&T=Modul e


Their main advantage is low power requirement, meaning you could make
a crystal set with only a ferrite rod aerial, and they didn't load
down the tuned circuit too much.

Derek