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From: jlg@lanl.ARPA (Jim Giles)
Newsgroups: net.sci
Subject: Re: Choice of voltage?
Message-ID: <177@lanl.ARPA>
Date: Fri, 7-Mar-86 13:22:14 EST
Article-I.D.: lanl.177
Posted: Fri Mar  7 13:22:14 1986
Date-Received: Mon, 10-Mar-86 08:25:09 EST
References: <drutx.107> <232@ccnysci.UUCP> <1199@mit-eddie.MIT.EDU>
Reply-To: jlg@a.UUCP (Jim Giles)
Distribution: net
Organization: Los Alamos National Laboratory
Lines: 15

>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....

As for -48 instead of +48 - well, they didn't really standardize on the
proper polarity for signal lines back in those days.  It was probably
just an arbitrary decision.

J. Giles
Los Alamos