Let's take the voltage first. The exchange 'battery' (or CO if you are in the USA) is set between -48 to -55v depending on where you are in the world, and to some extent, the type of exchange you are connected to. The figure of -48v is not set in stone. No need to worry if it is above this.
This battery voltage drops the minute a telephone is taken off of hook. The exchange 'equipment' senses the current flowing and drops the voltage to around 9 to 11 volts. The On-hook voltage stems from a day of long lines, electro-mechanical switching and phones.
They next point is that in an idle state a telephone line has a NEGATIVE potential with the EARTH being positive. Take out a voltmeter and set it so you can measure 50v DC. Place the POSITIVE probe to a known good earth and the NEGATIVE probe to the A and then the B legs respectively. (Ring & Tip in the USA). You will measure 48-55 volts on one leg, and around 4v on the other in relation to earth. The exchange battery has it's positive terminal put to Earth. Why? Simple. Corrosion. Positive earth systems don't suffer from corrosion (caused in a similar way to electro plating) in the same fashion as negative earth systems. Something the telephone companies worked out a very long time ago.
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