You have hereby declared yourself to be a proponent of dishonesty and vice.
Removing/replacing telephone junction box?
- Thread starter TheFunk
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The ringing is AC and used to be 80v I think. You got a hell of a belt if you shorted it out with your fingers when working on it!
For the British Post Office (BPO) and the Australian Post Office (APO) the voltage on a telephone line (from the exchange via two 200 Ohm windings on a relay) was nominally 52 V DC - with the Negative "Earthed" at the exchange.
Their standard for loop disconnect (Decadic) dialing was 10 impulses per second (with an allowable range 7 to 12) and a 67%/33% break/make ratio (allowable break range 63% to 72%).
If a telephone line is short-circuited and then disconnected (without the telephone connected), when the open-circuit occurs the "Back EMF" produced by the collapsing magnetic field in the balanced windings of the "A Relay" in the exchange equipment is quite high and will give "hell of a belt" if one's fingers are connect to the line at the time when disconnection occurs.
The Ringing AC Voltage was/is nominally 75 V at 17 2/3 Hz
However, this AC Voltage is deliberately NOT sinusoidal but is much "peakier" in waveform, so the "Peak" voltage is much higher than the 106 V peak to be expected from a sinusoidal 75 V AC waveform.
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Battery Positive is Earthed
Having a negative potential on cables relative to earth reduces the amount of corrosion of conductors.
I thought that theory was once used to justify positive-earthed cars, and was subsequently found to be utter nonsense?
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Yeah, I thought so, but I could be wrong.
Cars are different, because there is minimal connection of either side of the electrical system to true earth, hence no significant scope for leakages to earth (e.g. through moisture), so it doesn't make much difference what side of the electrical system is connected to the vehicle's metalwork. In other situations (e.g. telephone systems) where one side of the supply is connected to true earth and the prospect of such leakages does exist, electrolytic/galvanic corrosion will major on the 'anode' - hence a better situation if the non-earthed conductor is negative relative to earth.
Kind Regards, John
This anecdote might amuse:
I once bought an old bakelite box from a junk shop with a hand winder on it, and a length of plaited braid wire. On an impulse I tested it in the shop by putting the wires on my tongue and turning the handle. It worked
It was for calling the operator in the days before she was alerted automatically when you lifted the handset. I tried it out and it does indeed ring the bell downstairs. It could be useful perhaps for calling someone else to the phone from elsewhere in the house, but I'd be worried it might wake someone up at the exchange. It goes nicely with one of my bakelite phones, especially appropriately the one that doesn't have a dial.
I once bought an old bakelite box from a junk shop with a hand winder on it, and a length of plaited braid wire. On an impulse I tested it in the shop by putting the wires on my tongue and turning the handle. It worked
It was for calling the operator in the days before she was alerted automatically when you lifted the handset. I tried it out and it does indeed ring the bell downstairs. It could be useful perhaps for calling someone else to the phone from elsewhere in the house, but I'd be worried it might wake someone up at the exchange. It goes nicely with one of my bakelite phones, especially appropriately the one that doesn't have a dial.
My appologies.
It IS the posative terminal of a telephone exchange battery which is earthed, the opposite to the current practice in motor vehicles.
AIUI the ringing signal is also superimposed on the DC, which would make the peak voltage much higher than you'd expect
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