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why pstive terminal is grounded in telephone
- When you measure a phone line in your house, you will note there are two wires. Never touch any wires without checking their voltages first with an actual, working voltmeter. The main page of this article refers to these wires as tip and ring. Other people refer to them as lead and lag. It is easy to mix them up, so you should take care to label them. For the ring line, the usual reading is one of ground, or 0 volts DC. This is its rest state. But when someone is calling you up, the ring line is being driven, and if you are monitoring it with a voltmeter, you will note how it rises from ground to about 55 volts. The ring itself is usually cycling back and forth from ground to 55 volts, 60 times a second, so it is quite similar to AC. (In some parts of the world, it rises to 60 or 64 volts at peak; not to be outdone, some localities permit 48 volts as a peak state for the ring line.) Before you rely on this information to change the main page of this article, please look around for some reference material that you can cite. I have heard that the ring isn't always 60 times a second. In some locales, it can be quite a bit different, like 50 times a second. The pitch, tone, or timber will actually sound different, as a clapper is driven against a bell through electromotive force. 18.104.22.168 (talk) 08:17, 22 October 2011 (UTC)
Never, from my memory of being in the business, as high as 50 or 60 Hz. Ringer is 25 or 30. Telephone Circuits hobby page
- And I the writer of that is rather confused about many other things as well. "Lead" and "lag"? Never. In some countries, including Gt. Britain, it's common to refer to the wires as "A" and "B" (representing tip & ring respectively). And while there were a good many variations with various manual systems in the the past, on today's system the norm for most countries (including the U.S.A., Canada, the U.K. & Ireland, to name but a few) is to find approx -48 to -50V DC to ground/earth on the ring side of an idle line, and zero on the tip side. When a phone is in use, the loop current through the resistances feeding the line will cause the voltage as measured across the line to drop, with the ring-to-ground voltage decreasing and the tip-to-ground voltage increasing. The actual voltage across the phone when it is off hook will depend upon several factors, including the length of the line, but may be under 10V at the end of a long line, higher for a short line. In some systems, both now and in the past, a polarity reversal can be used to signal specific things, e.g. in the U.K. the 50V DC on the line reverses polarity immediately upon an incoming call reaching the line, then reverts to normal when the call is answered (in the past, a polarity reversal was common to signal answer supervision, i.e. that the called party had answered).
- Ringing is AC and most usually in the region of 20Hz up to about 95V (RMS) or so, but there are variations from place to place. There are a few countries which use 50 or 60Hz ringing, but they are relatively rare. In the days of multi-way party lines in North America, some telephone systems used different frequency ringing bursts with tuned ringers in order to provide selective ringing of the phones. A typical scheme employed five different ringing frequencies ranging from 20Hz up to 60Hz in order to provide fully selective ringing for 10 different telephones. Although it's not always done, in the majority of modern systems (and most of the older ones too for that matter), the AC ringing is superimposed on the DC battery on the line. However, in many cases (such as the old U.K. arrangement), this was done by applying the AC ringing supply on one side of the line to earth while maintaining the DC battery on the other side. Thus measuring from either side of the line to earth during ringing would show only constant DC battery or bursts of AC ringing, but a phone connected across the line would obviously see AC ringing superimposed on the 50V DC. 22.214.171.124 (talk) 21:24, 4 October 2012 (UTC)