Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Posting-Version: version B 2.10.3 Tandy Xenix 02/17/86; site gilbbs.UUCP Path: utzoo!watmath!clyde!burl!ulysses!bellcore!decvax!decwrl!pyramid!pesnta!hplabs!qantel!ptsfa!gilbbs!mc68020 From: mc68020@gilbbs.UUCP (Tom Keller) Newsgroups: net.sci Subject: Re: Choice of voltage? Message-ID: <56@gilbbs.UUCP> Date: Sun, 9-Mar-86 05:14:28 EST Article-I.D.: gilbbs.56 Posted: Sun Mar 9 05:14:28 1986 Date-Received: Wed, 12-Mar-86 22:02:11 EST References: <232@ccnysci.UUCP> <1199@mit-eddie.MIT.EDU> <177@lanl.ARPA> Distribution: net Organization: Gil's Place, Santa Rosa CA Lines: 25 In article <177@lanl.ARPA>, jlg@lanl.ARPA (Jim Giles) writes: > >Along the same lines, why -48 volts for telephone lines. I understand > >the negative voltage is to prevent loss of copper. But 48? > > The original phone system was run on battery power (just like the > telegraph). There were no AC power grids in those days (and not until > the light bulb). The value 48 is a multiple of the potential of the > lead acid batteries that they used (just as 6 and 12 are - you remember > car batteries). I don't know why 48 was used instead of 12 or 24 or.... I suspect that if you were to make the calculations, you would discover that 48 volts turned out ot be the most efficient compromise between boosting the voltage to achieve longer un-amplified signal runs, and I**2/R losses in the copper lines. Just a guess, mind you... -- ==================================== Disclaimer: I hereby disclaim any and all responsibility for disclaimers. tom keller {ihnp4, dual}!ptsfa!gilbbs!mc68020 (* we may not be big, but we're small! *)