25-11-2016, 03:51 PM
I would imagine that the ring wire or something similar would have been used in any telephone system that used Loop Disconnect (i.e.: Rotary Dial) dialling.
When you turn the dial, the first thing that happens is that a switch closes placing a short across the bell. This stops the dialling pulses making the bell tinkle as you dial.
Without that interconnecting wire, though, all the bells on other extensions on the same line would tinkle. In the days when the GPO, as it was then, owned all the telephone equipment and wiring which was then rented out to the subscriber, a sure sign when visiting someone's house that they had somehow 'acquired' another phone and wired their own extension was the characteristic tinkle from either phone if anybody dialled a number on the other one!
Anyone with a single phone could look inside the junction box and see that only two of the four wires was connected to the line and duplicate that for their illegal extension but, of course, they didn't know what the other wires were for or how to interconnect them between the two phones - which involved removing (different) links in both phones and knowing which wire to connect where!
Another catch for the unwary was that the bells were 500Ω impedance and up to four could be connected in series to give a total load of 2,000Ω
Modern phones have 2,000Ω bells and up to four can be connected in parallel resulting in the same 500Ω figure we started off with in the first place!
Inside a good old fashioned GPO telephone you would find two rows of terminals comprising, if my memory serves me correctly, 19 in total via which all the various parts of the phone interconnected.
In the simplified drawing on the left you can see the incoming line, bell capacitor, bell and dial shorting switch. When the dial is turned, the bell is shorted out do it can't tinkle. Note all the links!
Connecting two phones together is a bit trickier as you can see on the right. There is only one bell capacitor used and the two bells are connected in series but either dial switch shorts both bells.
To connect a third and fourth phone the same arrangement is used but with extra wires so that all of the bells are connected in series.
DTMF phones don't suffer from Bell tinkle, of course, so the bell wire has obviously been left out of some devices as Lawrence found out with his ADSL filters. As an alternative to finding a correctly wired filter, replacing the extension sockets with Master sockets would also have worked.
When you turn the dial, the first thing that happens is that a switch closes placing a short across the bell. This stops the dialling pulses making the bell tinkle as you dial.
Without that interconnecting wire, though, all the bells on other extensions on the same line would tinkle. In the days when the GPO, as it was then, owned all the telephone equipment and wiring which was then rented out to the subscriber, a sure sign when visiting someone's house that they had somehow 'acquired' another phone and wired their own extension was the characteristic tinkle from either phone if anybody dialled a number on the other one!
Anyone with a single phone could look inside the junction box and see that only two of the four wires was connected to the line and duplicate that for their illegal extension but, of course, they didn't know what the other wires were for or how to interconnect them between the two phones - which involved removing (different) links in both phones and knowing which wire to connect where!
Another catch for the unwary was that the bells were 500Ω impedance and up to four could be connected in series to give a total load of 2,000Ω
Modern phones have 2,000Ω bells and up to four can be connected in parallel resulting in the same 500Ω figure we started off with in the first place!
Inside a good old fashioned GPO telephone you would find two rows of terminals comprising, if my memory serves me correctly, 19 in total via which all the various parts of the phone interconnected.
In the simplified drawing on the left you can see the incoming line, bell capacitor, bell and dial shorting switch. When the dial is turned, the bell is shorted out do it can't tinkle. Note all the links!
Connecting two phones together is a bit trickier as you can see on the right. There is only one bell capacitor used and the two bells are connected in series but either dial switch shorts both bells.
To connect a third and fourth phone the same arrangement is used but with extra wires so that all of the bells are connected in series.
DTMF phones don't suffer from Bell tinkle, of course, so the bell wire has obviously been left out of some devices as Lawrence found out with his ADSL filters. As an alternative to finding a correctly wired filter, replacing the extension sockets with Master sockets would also have worked.
