In the early days, to call someone you picked up the telephone and told the operator the number of the person to whom you wanted to be connected.
Then the operator connected you. But soon telephones were built so that you entered the number yourself. Originally, you entered the numbers by means of a rotary “dial” as shown in Figure 1 (A). As you “dialed” the digits of the number, a switch connected to the rotary would pulse the DC loop-current. Figure 1 (B) shows that when the digit 3 was dialed, the loop-current was interrupted three times. The central office equipment would detect the pulses and decode the number.
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