In article ,
Frank Erskine writes:
The public network never had the 500 ohm bell coils in parallel in the
phone - they were always wired in series. Very occasionally private
circuits had the coils in parallel.
As you say though, in the old system the bells were connected in
series, up to (officially!) a maximum of four. Or was it six; and four
for a party line?

It was 4 for the longest line length supported.

The concept of REN didn't exist until the new system came about.

Back in the days of BT approvals in Baynard House, I took a couple
of devices along to get their approvals, which was a remarkably
unscientific process.

For the REN number rating, they supply a line simulating the longest
line length, and a (704?) standard phone on the end. The phone is
ringing. You plug in your appliance. If the phone stops ringing, you
get a REN of 4, and if it carries on ringing, you get a REN of 3.
If you have bought a second sample of your appliance, you are invited
to plug that in too. If the phone carries on ringing, you now get a
REN of 1.5. If you have bought a third sample of your appliance, you
can now try connecting that. If the phone carries on ringing, you now
get a REN of 1. You can carry on going if you have even more samples
with you. When I was at GEC, it was rare for us to have more than one
sample available to take along for testing, so most of our stuff got
a REN rating of 3, even though it was probably much lower. If you
look around at phones with REN ratings, you'll find they are normally
1, 1.5, or 3, and this explains why (e.g. there was no way to get a
REN rating of 2). The first caller display units I saw had a REN
rating of 0.25, for which they must have taken 12 of them along for
testing (although testing had been taken over by BABT by then, and
they might have changed the procedure).

--
Andrew Gabriel
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