Automatic telephone exchanges were first used in the 1900s in order to replace human operators and give them a well-deserved break.
In the developed countries these types of exchanges have been completely replaced by computerised ones, which are much more reliable and allow for additional services.
The idea for such an exchange belongs to Almon B Strowger, a funeral director in Kansas City, USA. The wife of a business competitor of his was a phone operator and she deliberately put through calls intended for Strowger to her husband. That, of course, got Strowger quite frustrated and he decided to put the human factor out of the equation.
The principle behind automatic exchanges, the electromechanical selector, is a very simple device, although it has been modified and improved many times. It works in the following way:
- When a current passes through an electromagnet, it pulls an anchor down.
- A tooth that is attached to that anchor contacts a cogwheel and turns it once.
- As the cogwheel turns, it connects your telephone to a different line.
This way if you want to select line number 7, you send seven electrical impulses to the selector. That is the familiar pulse dialing.
Such selectors usually have 11 possible outputs (one for each digit, and one for the 'busy' signal).
In a typical exchange there are level-one selectors, level-two selectors, and so on, according to its capacity, and each one takes a digit from the phone number you intend to connect to; then it automatically tries to connect you to a free selector on the next level.
When you have reached the last level and have sent the last digit to the exchange, you are connected to the other phone's final selector (unless it is in use, which results in a busy signal).
The final selector is a device with the following functions:
- It sends you a tone indicating that the other phone is ringing.
- It sends a 90V/25Hz AC signal to the other phone, which makes it ring.
- When the phone is picked up, it connects you and sends an impulse to the charging device (there ain't no such thing as a free lunch).
As there is a limited number of selectors, only a certain number of subscribers can dial a phone at a time. There are special devices, the Linefinder and the Allotter, that are used to find a free selector, and if one isn't available, you get a busy signal.
This is a very simplified explanation of how things work; exchanges based on this principle are surprisingly still widely used in some countries, in spite of the fact that they are very antiquated. So, if you ever try to dial a number while you're abroad and notice that tone dialing does not work, don't be surprised.