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National Geographic : 1947 Mar
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© National Geographic Society Ektachrome byWillard R.Culver When She Drops In a Coin, He Listens to See if the Signal Tells Clearly Whether It's aNickel, Dime, orQuarter Because service is speeded up if an operator knows unmistakably what coins are being dropped into a pay-station telephone, Bell Laboratories' scientists make careful tests of the signals' clarity. This experimental setup simulates a telephone officecircuit.
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