Automatic switching replaces the operator with a train of selectors that each move one step per dial pulse, so a caller reaches a number without anyone hearing the request. That removes the listening ear and brings in a different holder of power, the person who maintains the exchange and the written directory, while the human services of wrong-number repair and emergency routing must be rebuilt by rule. A two-digit step-by-step exchange reaches 100 lines and a three-digit one 1,000.
Part of Telecommunications
Operator-free switching by dial pulses and stepping selectors, covering how it works, what it costs to run, and how it shifts privacy and power.
Each Dial Pulse Moves A Selector One Step
A rotary dial interrupts the loop current at 10 pulses per second, with a centrifugal governor holding the rate (see Signaling and Ringing). Digit 7 is seven interruptions, 0.7 s at that rate. In a step-by-step exchange those pulses drive a selector directly: the first digit steps it up to a level, the second rotates it round the arc to a contact, and so on. Step-by-Step Switch gives the mechanics.
- Two stages with two-digit dialling: 10 × 10 = 100 lines
- Three stages with three-digit dialling: 10 × 10 × 10 = 1,000 lines
- A relay-built first selector needs 100 contact relays, 10 level-selecting relays and 10 position-holding relays: 120 relays for one stage, per the Step-by-Step Switch article
No central controller is needed, because each stage acts on the pulses it receives. Blocking happens at a stage whose ten outlets are all busy; the caller hears a fast busy tone and must redial. The same article suggests sizing each stage to keep blocking below 1% at peak.
The Story Of Its Invention Is A Story About Who Held The Switch
The corpus tells the origin of the step-by-step exchange this way: a Kansas City undertaker, Almon Strowger, suspected that telephone operators were routing calls to a rival, and the switch of 1889 needed no operator at all. Whether or not every detail holds, the story shows what an operator is: the person through whom a business or a household reaches anyone else. Removing the person removes that dependency.
What Changes For A Settlement
Privacy gains. No one hears the request, and a plain step-by-step exchange has no memory of who called whom unless someone adds a counter or recorder (call statistics are hard to collect, per Switching Systems). The contact pattern that Manual Exchange puts in one head is simply not collected.
Power moves rather than vanishes.
- Whoever maintains the exchange can still attach test gear to a line. Impedance Matching notes that a high-impedance bridging test set can be attached in service without interrupting a call.
- The directory becomes the centre of the system. Whoever writes and assigns the numbers decides who can be reached.
- Skills concentrate. A step-by-step exchange can be maintained by a competent non-specialist, per its article, but a settlement then depends on that person for every fault.
Services must be rebuilt by rule. Switching Systems lists what the operator did: fixed wrong numbers, resolved a name into a number, announced the time and routed emergencies. An automatic exchange does none of these. Settle three things in writing: a number everybody can dial for help, a published directory with a named keeper, and a rule about who may test or open the exchange. For the principles, see Data Access and Accountability Mechanisms.
When To Automate
Switching Systems says to build the manual board first and automate only when growth makes the operator burden intolerable. It puts step-by-step at roughly 10 to 200 subscribers, above which the multiple selector stages become a heavy maintenance load, and crossbar at 100 to 10,000. A settlement of a few dozen lines gains little from the machinery.
What You Need
- Selector mechanisms or relays and magnets to build them, with bank contacts and wipers
- A rotary dial or a pulse-tapping key producing 10 pulses per second
- Busy and ring tones and a ringing generator at the final selector
- A written directory and a numbering plan
- A maintainer who can adjust ratchets, clean bank contacts and straighten wipers
Common Mistakes
- Automating at 20 lines. The stepping hardware costs more than the operator it replaces.
- No operator fallback. The first wrong number or emergency has nobody to ask.
- Worn ratchet teeth ignored. Dialling 7 connects to 6 or 8; clean and replace, per Step-by-Step Switch.
- A dial running fast or slow. Pulses outside 10 per second ± 1 produce wrong connections.
Safety
Exchange hazards
A step-by-step exchange runs from the central battery, commonly 48 V (see Battery and Powering), and rings with 90 V RMS at 20 Hz in the North American standard. Switch banks have many moving parts, so keep hands out while powered, and mount fuses on every battery feed.
What’s Next
- Learn next: Network Architecture, joining several exchanges
- Combine with: Crossbar Switch when common control becomes worthwhile