Tip and ring

Tip and ring are the names of the two conductors or sides of a telephone line. The terms originate in reference to the telephone plugs used for connecting telephone calls in manual switchboards. One side of the line is connected to the metal tip of the plug, and the second is connected to a metal ring behind the tip, separated and insulated from the tip by a non-conducting material. When inserted into a jack, the plug's tip conductor connects first, followed by the ring conductor. In many European countries tip and ring are referred to as the A and B wires.

The ring conductor has a direct current (DC) potential of −48V to −52V with respect to tip conductor when the line is in the on-hook (idle) state. Neither conductor is referenced to ground. Floating both conductors (not referencing either one to ground) minimizes the pickup of hum from any nearby alternating current (AC) power wires.

The terms tip and ring originated in the early days of telephony when telephone operators used plugs to connect customer calls. They are named after the parts of the plug to which the wires were connected. The words are often abbreviated as T and R.

The telephone company maintains large battery systems that supply DC line voltage for the operation of analog telephone service ("plain old telephone service", or POTS) at customer locations. The voltage supplied is a compromise between operational needs for reliable service and safety precautions for customers and service personnel. The length of the line to a customer telephone interface presents a resistance across which the central office voltage experiences a drop and therefore the voltage at the customer site may vary. The nominal battery voltage is 52.1 V, based on a 24-cell lead-acid battery. The voltage at a subscriber's network interface is typically 48 V.

In the middle 20th century, long loops in many rural areas of North America used range extenders, which operated at 100 or 130 volts to ensure reliable signaling. Some rural switching systems were designed to apply range extenders internally and thus share a few extenders among many lines, while for other lines, one extender was applied externally per line.

Originally, the potentials on the wires were positive with respect to earth (ground). Telephone companies discovered that with positive voltage on the copper wires, copper wires experienced corrosion due to electrolysis. Operating in reverse, with a negative potential on the wires, the copper is protected from corrosion by cathodic protection.

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