Strowger's switch¶
Almon Strowger was an undertaker in Kansas City, not an engineer. The popular story says he was convinced that a local operator, the wife of a rival undertaker, was sending his customers' calls to her husband. Whether or not that is true, he patented an automatic telephone exchange in 1891. It needed no operator at all.
His step-by-step (or Strowger) switch was electromechanical. Each digit the caller sent moved a selector arm up and around to the next stage, so each stage of switches selected one digit of the number. The switch was driven directly by electrical pulses from the subscriber's phone.
Pulse dialing¶
Early automatic phones used push buttons to generate the pulses, which were clumsy. The rotary dial, refined in the late 1890s and early 1900s, became the standard way to send them.
How it works:
- Off-hook, the line carries a steady DC loop current from the central office.
- The caller turns the finger wheel to the stop and lets go.
- A spring returns the wheel, and a governor keeps the return speed constant.
- On the way back, a cam opens and closes a switch contact, interrupting the loop current once per pulse.
- Dialing 1 sends one pulse, 5 sends five, and 0 sends ten.
The standard rate is about 10 pulses per second, with each pulse open for roughly 60% of its cycle and closed for 40%. The pause while the dial is being wound for the next digit marks the boundary between digits.
The technique is called pulse dialing, loop-disconnect dialing, or decadic dialing. Because the dial only opens and closes the loop, the same pulses can be made by tapping the hook switch quickly and rhythmically. That is "switch-hook dialing."
Bigger and better switches¶
Step-by-step switches were noisy, mechanically busy, and hard to scale for large cities. Successors included:
- Panel switch (1920s): used in large U.S. cities, with motor-driven selectors.
- Crossbar switch (1938): a grid of horizontal and vertical bars. Setting one crosspoint connected two lines. Crossbar used common control, meaning shared logic that set up the call and then released, which made it faster and more flexible. Crossbar was the main switching technology through the 1950s and 1960s.
The word "crossbar" survives in computing, where a crossbar switch still describes a grid interconnect in CPUs and network switch fabrics.
The classic desk phone¶
The Western Electric Model 500, introduced in 1949, is the image most people have of a rotary phone. It was heavy, very durable, rented rather than sold, and designed with careful acoustic engineering. Tens of millions were made.
In Canada the equivalent was built by Northern Electric, Bell Canada's manufacturing arm. Many surviving Northern Electric sets still work today with little more than cleaning and a new cord (see Using a rotary phone today).
The legacy¶
Pulse dialing was gradually replaced by Touch-Tone after 1963 (see The digital network). Most central office switches kept accepting pulses for decades for backward compatibility. The word "dial" survives in everyday language, even though almost no one has used a dial to make a call in a generation.