Path: utzoo!dciem!nrcaer!scs!spl1!laidbak!att!pacbell!ames!husc6!bloom-beacon!westmark.UUCP!dave
From: dave@westmark.UUCP (Dave Levenson)
Newsgroups: comp.dcom.telecom
Subject: Re: Three wire lines (was 2 line wiring)
Message-ID: <196@westmark.UUCP>
Date: 4 Jun 88 03:43:48 GMT
Article-I.D.: westmark.196
References: <8805262016.AA02325@kestrel> <14528@tut.cis.ohio-state.edu>
Sender: daemon@bloom-beacon.MIT.EDU
Organization: Westmark, Inc., Warren, NJ, USA
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In article <14528@tut.cis.ohio-state.edu>, mdf@TUT.CIS.OHIO-STATE.EDU (Mark D. Freeman) writes:
> 
> My house was built in the 1920's and was wired for telephone with 3
> wires.  I can understand two or four, but three?  Why?

In those days, two-party telephone lines were common.  Your phone
and someone else's phone shared a pair of tip/ring wires.  Only one
of you could actually use the line at a time.

For incoming calls, one of your numbers caused ringing to be applied
between tip and ground.  The other number caused ringing to be
applied between ring and ground.  The bells in your phones were
wired between the appropriate side of the line and ground.  Thus you
need three wires to each set: tip, ring, and ground.  The third wire
(usually colored yellow) was probably connected to the ground
terminal of your protector.

For outgoing calls, the party who rings tip to ground was wired so
that when the set is off hook, one coil of the set ringer was
connected between tip and ground, without the usual blocking
capacitor in series.  This high-impedence, low-resistance ground on
the tip side of the line could be detected by the central office for
originating party identification.

-- 
Dave Levenson
Westmark, Inc.		The Man in the Mooney
Warren, NJ USA
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