On Fri, 15 Feb 2008 20:32:51 GMT, wrote:

On 15 Feb,
"Roger Mills" wrote:

ISTR that old bells had an impediance of 1k ohms - thus having a REN of 4 -
whereas later ones with a REN of 1 had an impedance of 4k ohms. So,
basically, you need to wire a 3k ohm (some say 2.2k - not quite sure why)
resistor in series with the bell, and you're sorted.

Some had two coils, which could be wired in series for 4K and parallel for
1K.

The old extension system wired the bells in series and required the coils in
parallel. the newer plug (modern type sockets) have the bells in parallel,
and require the coils in series.

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?
The early 'plan 4' system of plug and socket telephones was a bit of
fun. There had to be at least one fixed bell (usually a separate
bellset was permanently connected) and each socket had to have a break
jack to maintain continuity of the bell circuit when a phone was
plugged/unplugged.

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

--
Frank Erskine