2. Theory of Electromechanical Switching

The Invention of Automatic Switching

Almon B. Strowger was an undertaker in Kansas City, USA. The story goes that there was a competing undertaker locally whose wife was an operator at the local (manual) telephone exchange. Whenever a caller asked to be put through to Strowger, calls were deliberately put through to his competitor. This obviously frustrated Strowger greatly and he set about devising a system for doing away with the human part of the equation !

Strowger developed a system of automatic switching using an electromechanicalcal switch based around around electromagnets and pawls. With the help of his nephew (Walter S. Strowger) he produced a working model in 1888 (US Patent No. 447918 10/6/1891). In this selector, a moving wiper (with contacts on the end) moved up to and around a bank of many other contacts, making a connection with any one of them.

Strowger did not invent the idea of automatic switching; it was first invented in 1879 by Connolly & McTigthe but Strowger was the first to put it to effective use. Together with Joseph B. Harris and Moses A. Meyer, Strowger formed his company 'Strowger Automatic Telephone Exchange' in October 1891.

In the late 1890's Almon B. Strowger retired and eventually died in 1902. In 1901, Joseph Harris licenced the Strowger selectors to the Automatic Electric Co. (AE); the two companies merged in 1908. The company still exists today as AG Communications Systems (www.agcs.com), having undergone various corporate changes and buyouts along the way.

Selector Theory

A selector starts in the 'home' position and with each 'impulse' the wiper contacts would progress round the output bank to the next position. Each output would be connected to a different subscriber, thus the caller could connect to any other subscriber who was connected to that bank, without any manual assistance from an operator.

Figure 2 - Diagram of a simple Selector

In Figure 2 (above), the selector has 10 outputs, so a caller can choose to connect to any of 10 different subscribers by dialling any digit from 1 to 0 (0=10). This sort of automatic selector is known as a Uniselector, as it moves in just one plane (rotary).

By mounting several arcs of outlets on top of each other, the number of outlets can be increased significantly but the wipers are then required to move both horizontally to select a bank and then vertically to move around that bank to the required outlet. Such a selector is known as a Two-Motion Selector. Two-motion selectors typically have 10 rows of 10 outlets, thus 100 possible outlets altogether. A two-motion selector can therefore accept two dialled digits from a subscriber and route the call to any of 100 numbers. The selector 'wipers' always start in their resting 'home' position. The first digit moves the selector veritically up to the corresponding level and then the second digit moves the wipers around the contacts of that level. This is shown in figure 3, below.

Final Selector
Figure 3a - A Two-Motion "Final" Selector

To see an animated version of the above diagram, click HERE!

The type of selector shown above is known as a Final Selector as it takes the final two digits of the number dialled. Most numbers dialled are several digits longer, and therefore pass through a chain of selectors. Selectors previous to the Final Selectors are different; they are called Group Selectors. Group selectors take only ONE digit from the caller, and step up the number of levels according to the digit dialled. The rotary movement is then automatic; the wipers search around that level to find a free outlet - i.e. the next free selector in the chain. This is covered in more depth later.

Group Selector
Figure 3b - A "Group" Selector and a 'bank' for it to slot into

The Rotary Dial

In Strowger's system, selecting digits to dial was done by a complicated system involving five separate wires. Later, the system of Timed Pulse (TP) dialling was inventned using a rotary dial. With TP dialling, only one pair of wires is required for a telephone, the speech pair. To dial a digit, the circuit is interrupted according to the number dialled so, for example, if you dialled a '4' then the line would be pulsed four times, quickly in succession. After a moment, it was assumed that the digit was complete and that any further pulses belonged to the next digit. In order to ensure that successive digits didn't come too soon and thus be mistaken for pulses belonging to the previous digit, the finger stop on the dial was put some way round so that after removing your finger from the dial, there was a minimum time taken for the dial to return to the home position. It is important to note here that for the purposes of dialling, the digit '0' sends TEN pulses for dialling - i.e. the selector will step around to the 10th position.

Progress Tones

With manual switching systems, there had always been an operator to advise the caller of the current status. Having removed the need for an operator, a system was required to indicate call progress to the caller. A series of distinctive tones was developed which were produced by a machine called a Ring Generator. The ring generator was entirely electromechanical; different cadences and tones were produced by rotating cams connected to a generator. As well as generating the tones, the Ring Generator machine also provided timed pulses which were used by various processes throughout the exchange. The progress tones produced were as follows :

  • Dial Tone (DT). This is a 33 c/s continuous note and is applied to the line after the subscriber has lifted his handset and the switching equipment has allocated him an available outlet for this call to proceed. There would have been a physical limit on the number of calls an exchange could handle so if all equipment was already in use, the subscriber would not get dial tone. The actual pitch of the dial Tone varied from exchange to exchange depending on the adjustment of the ring generator.
  • Busy Tone (BT). A higher pitched note of 400 c/s interrupted to give a cadence of 0.75 seconds on, 0.75 seconds off. Busy tone indicated either that the called subscriber is already off-hook (busy) or that the route to the called subscriber is congested. In later systems, a slightly different cadence was introduced in order to distinguish between these two scenarios.
  • Number Unobtainable Tone (NUT). Identical pitch to the busy tone but continuous. This tone is used to indicate that a number is out of service, faulty or that a spare line has been dialled.
  • Ring Tone (RT). A tone of 133c/s which interrupted in the same cadence as the ring current which rings the telephone's bell at the called party's end : 0.4 seconds on, 0.2 seconds off.

You can hear original recordings of these tones elsewhere on these pages.

The first public exchange

The first public automatic telephone exchange in the UK opended in Epsom, Surrey (England) in 1912. As mentioned previously, manual switching required an operator for every call and thus was expensive on manpower, however, when first developed, automatic switching systems were comparitively expensive and in the Post-war period (1914 onwards) female labour was cheap, so the advanages of moving to an automatic system were not great.

PABX4 Catford

The above photo is of a working Strowger exchange; it was working until July 1995 at a public building in Catford and then replaced by a modern exchange. This Strowger exchange took up a large room - its replacement was the size of a filing cabinet. The complete working exchange was carefully removed by members of the Telecomms Heritatge Group who intend to preserve it and reassemble it as a working exhibit somewhere. Thanks to Martin Loach for the photo.

In the forground of the photo are the lead acid batteries; all Strowger exchanges had battery power supplies to protect them in the event of a power cut.

In the rest of the picture you can see the racks of selectors. This exchange was actually a PABX (Private Automatic Branch Exchange) which is an internal exchange used for handling calls within a building, rather than across the PSTN. Outside access onto the PSTN was provided of course, normally by dialling a '9'. All Strowger exchanges were very noisy places with hundred of selectors constantly clicking away.

Section 3 : Numbering Schemes
Back to the Main Index

This page is sponsored by SEG - Our Sponsors SEG Communications

(C)1996 - Reproduction in part or full without permission is prohibited.