Physics – Why did the first phones use a rotating dial instead of 10 individual buttons?

The most important factor is that at the time of the invention of the phone, the automatic routing of calls did not exist. There were people sitting in central switching stations. You told them, "Connect me to Mrs. Johnson," and they reconnected the wires – and now you're connected to Ms. Johnson. Later, automatic call routing was invented, but do not have use dial tone yet – it would have been too complicated to handle for simple electronics, because the switching system relied mainly on relays. Instead, he used pulse dialing, which is much simpler. It can even be mechanical (learn more about rotary selectors).

Consider that the difference between pulse dialing and tone dialing does not only exist on the end user side, or only concerns the user experience. You must also consider various other factors:

  • current technology available
  • cost of technology
  • existing infrastructure
  • availability of qualified personnel to maintain it.

At the time, telegraph were of common use. Generally, these have worked mechanically. Later versions have been upgraded with electronics, but it is of lesser importance. To perform an automated call routing based on pulse dialing, you can use mechanical (and later electronic) components similar to those already used in telegraphy. These allowed for cost savings, faster introduction into the utility and easier maintenance.

Toneon the other hand, requires relatively good filters and detectors to match the tone frequencies. It is also harder to generate tones than pulses. As electronics improved and tone dialing became feasible, its introduction followed, as tone dialing is generally less prone to noise and errors than simply counting. clicks ".

For the current user interface, let's look at its history.

Before transistor switching, 95% of the configuration of electrical appliances was realized using:

  • switches: stable (press-stay), unstable (press-return), one-of-many (press one, reset others)
  • potentiometers (as well as similar devices: variable resistances, variable capacitors, inductances …)

So there were devices "on / off" and "buttons": on / off to connect or disconnect the power, a switch 1-2-3-4-5 to change channels and a "button" to adjust the volume or tune a circuit. There was no "pushing this to start a process that makes A, B then C".

And of course, there were "sliders": buttons and sliders. In other words, all the traditional potentiometers and others.

Currently, we all know the idea that if you press a certain button, a machine will start and if you press this same button 5 times, the machine will perform its action with 5 levels of increasing intensity. So, if you want a "louder sound", you press the "+" button X times. It's easy. But this requires a complex circuit that counts the presses, plus a DAC that converts the count into a particular voltage or capacity, and so on.

At the time, if you wanted to change the intensity of something, you can either:

  • pressed 1-2-3-4-5 -…- X selection buttons (stable selection)
  • turned a knob left or right to get "more" or "less"
  • drag a slider up or down to produce "more" or "less".

Buttons and sliders set resistance / capacitance / inductance directly. These were simple mechanical devices.

The first thing that comes to mind is the pulse dialing, for which you need to emit more or fewer pulses. Suppose you need to emit a given number of impulses. So we will take a tube filled with N contacts and a metal ball: if you drop the ball in the tube, it will "connect" exactly N times. Take 10 tubes, each with a different number of contacts, and you can easily emit 1 to 10 pulses by simply dropping a ball in the right tube. It's easy. This is not expensive. It works. Maintenance is trivial. There are no complex or expensive circuits. But it is also harder to use for the end user. A rotating machine with automatic spring return would be much better, and almost as cheap!

Of course, this story is only my guess: I do not know what was the exact reasoning of Bell Labs, etc. I simply made an educated guess based on my own knowledge of technology.