Pulse dialing

Content sourced from Wikipedia, licensed under CC BY-SA 3.0.

Pulse dialing is an old way to dial numbers by briefly interrupting the telephone line. The signal is created by opening and closing the loop in a precise pattern, which tells the telephone network which digits you are dialing. This pattern is also called a pulse train or loop-disconnect dialing.

Digits are sent using pulses. In the common decadic system, the digits 1 through 9 are sent as 1 to 9 pulses, and the digit 0 is sent as 10 pulses. Most countries used this same idea, though a few had different mappings.

The pulse rate is about ten pulses per second, but early systems varied. In the United States, equipment was allowed to be accurate around 8 to 11 pulses per second; in the United Kingdom, about 7 to 12 pulses per second. The speed was limited by the mechanical parts of the switches.

How it worked: when you turned the dial, it created a sequence of openings and closings as the dial returned to rest. The telephone exchange read these pulses with relays or digit registers to determine the number you dialed. The need for reliable mechanical timing kept the rate around ten pulses per second.

A lot of this came from the rise of automatic exchanges in the late 19th and early 20th centuries. The rotary dial used a moving wheel to generate the pulse trains, and early systems often used two or more wires to carry the signals. Over time, technology moved toward simpler two-wire systems and more advanced switching.

Push-button dialing and tone signaling eventually took over. In 1963, the Bell System introduced touch-tone dialing (DTMF), and key-based dialing became common. Rotary dialing gradually faded from new phones, though many systems still support it. Modern mobile phones and most voice-over-IP services use different signaling methods, often sending the number as part of the call setup rather than as pulses.

Some phones still let you hear the clicking of the pulses, but designers often reduce or eliminate this noise. In the past, people even tapped the switch hook to dial quickly, including on coin-operated boxes in the UK. Today, dialing is usually done with tones or digital signaling, but pulse dialing is an important part of telephone history.