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This is the classic supervision test.

This type of supervision test was found in most #1 and #5 crossbar offices. This is my recreation of it, based upon memory and from listening to the Evan Doorbell tapes. Since there was a snarky comment about lack of explanation, here it is, copied and pasted from an answer I made to a previous inquiry,: In the days of the old electro-mechanical telephone network, most exchanges of any size had these test numbers. The term "supervision" comes from the old operator cord boards on which there was a lamp for each line. When that line was in the "off-hook" or "answered" condition, the supervision lamp on the operator's board would light up, letting her know the line was in use. As time progressed, and after Direct Distance Dialing (DDD) came out, then people could dial their own long distance calls. In order to keep track of how long they talked and then to bill them accordingly, some automatic way to monitor the "supervision" of every line had to be implemented. ---------------------------------------------------------------------------------------------------------------------------------- In this way, the old "supervisory" lamp circuits for operators slowly got redefined and used for billing. In a step office such as mine, when a called line answers and goes "off-hook" the line DC polarity reverses. This is known as "reversed polarity supervision". It can be seen in this video on the ammeter which monitors the testboard's outgoing line. That meter has zero at the center. RIGHT of center is STRAIGHT line polarity with Tip being positive and Ring being negative. When the meter needle passes through zero and goes left of center, that is reversed polarity. Remember it this way "Left is the REVERSE of normal". ------------------------------------------------------------------------------------------------------------------------------------- So a supervision test is a number that when called acts like someone picking up and hanging up a phone. Its purpose is to test the action of the supervisory relays. Hope this helps explain. Please go look up Evan Doorbell and all of his old phone tapes. He explains it all in much greater detail. Keep the questions coming guys! I am happy to answer them all!