Re: Why -48V for telephone lines, and not positive?

From: BFoelsch (BFoelsch_at_comcast.ditch.this.net)
Date: 12/01/04 

Date: Wed, 1 Dec 2004 13:38:34 -0500

"Winfield Hill" <hill_a@t_rowland-dotties-harvard-dot.s-edu> wrote in
message news:coisj00l0a@drn.newsguy.com...
> JeffM wrote...
> >
> >> when this stuff was invented
> >
> > Yup. Stroger Switches were invented in 1884--by an undertaker.
>
> Do you think they used -48V back then?

I doubt that there was any standard in the days before electrical
amplification. My guess is that for a long line the voltage might be a good
deal more than 48 volts.

I looked in my old college text on Telephony (Albert, Fundamentals of
Telephony, Mc-Graw-Hill, 1943) and found no discussion on the polarity
issue, but he did offer that magneto (crank) telephone systems used 3 dry
cells per set, which would of course work out to 4.5 volts. However, these
sets coupled to the line through a transformer so the line carried purely
voice current an ringing current with no DC potential. He then goes on to
say that most central battery systems used 24 volts, except for dial and
toll offices, which used 48. So, I guess that as late as 1943 24 volts was
the standard.

He makes no mention of the polarity issue, but I picked through a lot of the
schematics of both dial and manual systems and they all show the
common-positive system.

>
>
> --
> Thanks,
> - Win

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