top of page

The TOC below is a content outline of this website with links.
Use the search feature for more detail. Check out the hierarchical TOC graph.   

Table of Contents for Calling315.com

https://www.calling315.com/Home page: This website explores the golden age of electromechanical telephone exchanges with six hands on interactive demos. Using over 100 pieces of media and many pictures, users can interact with five relays and see their place in telephone system history.

There are also ~50 explainer sections covering 15 telephone switch types, exchange technology and methods. Of interest too are the explainers for SxS, 7A Rotary, Panel and the #5 Crossbar exchanges.

With over 250 images and 25 videos this site offers an introduction for those interested in electromechanical telephone systems.

https://www.calling315.com/all-relayThe all-relay telephone exchange only used relays to make the talking path connection. There were two versions, one by the North Electric Company and one by Berulander and Palmgren of R.A.T Company. This page explains both systems and compares them.    

                                                                   

https://www.calling315.com/basics-of-telephone-exchangesInformation on the basics of electromechanical telephone exchanges. Including Panel, 7A Rotary, Crossbar and Step-by-Step (Strowger) switching methods.

All the key components are outlined including batteries, switching fabrics, common control,   dial control. Exchange architecture is explained with demos of exchange parts with explainer videos and many pictures

https://www.calling315.com/common-controlMany electro-mechanical telephone exchanges use "common control" for controlling switching operations. Examples are provided for Panel and Crossbar systems using senders and markers. Common control is compared to "progressive dial control" for SxS exchanges

https://www.calling315.com/connolly-switch-1The Connolly brothers and McTighe patented the first dial driven switch for a telephone exchange in 1879 with images. This section is an explainer

https://www.calling315.com/creditsCredits for the development of the Tribute to Telephone Relays demo device.

https://www.calling315.com/crossbar-exchangeExplainer for how the Number 5 Crossbar exchange works. 

https://www.calling315.com/crossbar-switchThis page explains the invention of the telephone crossbar switch. Reynolds and Betulander's crossbar invention are explained. History, videos, images

https://www.calling315.com/demo-1An interactive demo for appreciating the value of telephone relays and electro-mechanical, vintage exchanges. This is a six min voice narration explaining the brief history of telephone exchanges and their relays. 

https://www.calling315.com/demo-2Sound codes: Anything a modern computer can do, its relay-based cousin can do. Just much slower. Many operations need > 5 relays so the examples in this demo are illustrative only. The Demo sounds are meant for fun. Explore 31 combos of relay operations, each combo playing a different sound. Fun for kids. 

https://www.calling315.com/demo-3Central Office Equipment Sounds. Listen to nine recordings of sounds recorded inside a CO. 

https://www.calling315.com/demo-4An intercept message is a recorded announcement concerning the status of the call. These messages were used extensively in the Bell System. Listen to the famous voices of Mary Moore, Jane Barbe, and Pat Fleet.

https://www.calling315.com/demo-5Telephone themed remixes. Listen to Madonna, Jane Barbe, Lily Tomlin, Evan Doorbell, “relays dreaming” and more. The pieces are a fusion of telephone system sounds, voices, messages and music. A rare genre for those who appreciate vintage telephone systems. 

There are also seven other Evan Doorbell remixes. Contact the author for the secret access code. 

https://www.calling315.com/demo-6Dancing to the music -- It's a fun take on relays jiving to the music. 

https://www.calling315.com/demosExperiment with six interactive demos related to electromechanical telephone relays. With over 100 pieces of media learn about the golden age of relay-based telephone exchanges. This is an index to the six demos. 

https://www.calling315.com/diy-telephone-systemDIY STEM 8-line automatic telephone system. 54 relays and 4 rotary selector switches with line finder and final connector. Explainer for this hand built system . Entry in San Francisco Science Fair in 1960's. 

https://www.calling315.com/eb-craftExtraordinary engineer E.B. Craft invented the flat spring telephone relay and became the president of Western Electric.

https://www.calling315.com/ericsson-500-switchExplainer for this 500 point polar coordinate telephone switch. It was used in electromechanical exchanges worldwide

https://www.calling315.com/ericsson-xy-switchA 100 point electro-mechanical switch to replace a stronger type in a SxS office. It had 2 degrees of linear motion so called the XY coordinate switch. It was originally designed by LM Ericsson in Sweden. Stromberg Carlson licensed the design for use worldwide. 

https://www.calling315.com/evan-doorbell-remixesEvan Doorbell, a pseudonym, was a 1970's explainer of electro-mechanical exchanges and their sounds and operations. He has a golden voice and recorded many hours of calls on panel, sxs and crossbar offices

https://www.calling315.com/exchange-anatomyThe world of vintage electro-mechanical telephone exchanges is explored. Explanations of Panel, 7A Rotary, Step-by-Step and Crossbar is provided with many pictures and explainer videos.

~50 different explainer topics are explored. This is the main page that has links to many other topics using a graphical "map". 

https://www.calling315.com/exchange-anatomy/floorplanThe basic ideas for how a call is made. Several "floorplans" are outlined showing the switch hierarchy for making a call. Starting with a 1-digit system, then 2-digits, then 3-digits, the switch train is explained. 

These ideas are the foundation for how Panel, 7A Rotary Crossbar and Step-by-Step systems are designed. 

https://www.calling315.com/exchange-anatomy/rotary-7a-exchangeExplainer for how the 7A rotary telephone exchange works with images and videos. Demo of the Ferrymead exchange system at Christchurch, New Zealand. 

https://www.calling315.com/fabricsFabrics are a network of switches and telephone systems depend on them for making the talking paths. The crossbar switch is used an example of building a network fabric for an exchange. 

https://www.calling315.com/fallwahler-switchA telephone switch developed in Germany 1920's. Similar to the panel switch, motor driven, used gravity and a governor to time the brush stopping synced with the dial pulses. Known as the "falling selector" switch

https://www.calling315.com/gallery-of-closeupsCloseup pictures of 8-line DIY 8-line telephone system built in SF in the 1960's, STEM project with 54 relays and 4 stepper switches. Was displayed at the San Francisco Science Fair at the Academy of Sciences in Golden Gate Park. 

https://www.calling315.com/keiths-strowgerAlexander Keith reinvented the first Strowger telephone exchange switch. These pages explain the basics of his switch design and how it improved on Strowger's original design. This design became the workhorse for SxS systems worldwide.

https://www.calling315.com/line-circuitThe Line Circuit is essential for all electro-mechanical telephone exchanges. It sits at the edge of the exchange and provides the logic to connect the phone to the switching system. It uses the Line (L) and Cutoff (CO) relays

https://www.calling315.com/lorimer-switchThe Lorimer brothers patented important switch technology for automatic telephone exchanges. These ideas others leveraged to build more practical designs. 

https://www.calling315.com/moreThe more button leads to a menu for; contact; credits; motivation, search and the Teleharmonium all related to electro-mechanical telephone switched systems

https://www.calling315.com/motivationRead about the motivation for designing and building the 5-relay based demo device. Also, explore how vintage telephone exchanges worked with many reviews of telephone switches including panel, crossbar, rotary, Strowger type and 10 other less popular switches.

https://www.calling315.com/number-5-crossbar-diagramNumber 5 Crossbar block diagram in high resolution. See the details for a complete electromechanical telephone exchange.

https://www.calling315.com/one-hundred-anniversary-panel-nycThe first 7-digit electromechanical Panel telephone exchange was installed in NYC Oct, 1922. This is an explainer article with images.

https://www.calling315.com/other-elementsCoverage of components for electro-mechanical telephone exchanges including; ringing machines, wiring and structural support, power rooms, backup,  and battery rooms and testing methods to keep the exchange working 24x7. 

https://www.calling315.com/panel-exchangeExplainer for how a panel telephone exchange works using images and videos. 

https://www.calling315.com/panel-switchThe panel telephone switch is explained with pictures and videos. This switch was a workhorse in metro exchnages in North America from ~1921 until the 1960's.

https://www.calling315.com/peculiarThis page reviews odd and pecular methods to make switch connections for an automatic telephone exchanges. All these methods were not successful but advanced the state of the of art for exchange switching.

https://www.calling315.com/power-roomThis page explaines the equipment need to operate a vintage telephone exchange power room including AC to DC generators for charging, backup gas powered generators and large 77 ton 48 volt battey systems.

https://www.calling315.com/relay-logicTelephone relays are explained with pictures. Coverage of type E, R, U, UB and wire spring types. Simple circuits are provided showing the value of a billion relays used in electro-mechanical telephone systems.

https://www.calling315.com/ringing-machinesTelephone exchanges needed ways to generate ringing voltage, call-progress tones and interrupters for ringing and busy tones.

https://www.calling315.com/rotary-7aWestern Electric 7A rotary switch explainer. Coverage and video demos of the Ferrymead (Christchurch NZ) display system.

https://www.calling315.com/rotary-phonesShort summary of the first rotary dial phones. A few pictures of classic phones and some reimagined telephones including one designed by Leonardo DaVinci and one from Einstein.

https://www.calling315.com/rotary-solo-stepperUnderstand the basics of rotary stepper switches. A clear explanation of their operation and how they are used in small telephone exchanges (PBX). See a short video on a stepper in operation. Plus a video showing a relay tester appliance.

https://www.calling315.com/sci-fair-signageTwo signs from the 1960's SF Bay Area Science Fair. They provided demo information on how a DIY 8-line telephone system operated. STEM project

https://www.calling315.com/search-boxSearch this site for words

https://www.calling315.com/sequence-diagramA relay sequence diagram for a 8 line DIY telephone system, STEM, with 54 relays and  stepper switches

https://www.calling315.com/sequence-switchSequence switches are explained as used in the 7A rotary and panel telephone exchanges. Invented by F. McBerty of western electric. With explainer videos. 

https://www.calling315.com/strowgers-first-switchAlmon Strowger built the world's first practical telephone switch and exchanges. This page explaines how the switch works, examines Strowger's patent for it, reviews how it was different from previous switches.

https://www.calling315.com/switchesThis page explores 15 different core (plus derivatives) telephone exchange switch types including: 7A Rotary, All-relay, Connolly brothers switch, Crossbar, Ericsson 500, first Strowger, Fallwähler Switch, Keith's plunger, Keith's Strowger (SxS), Lorimar switch, Panel, Sequence, Solo rotary, Ericsson/Stromberg Carlson XY, Zither.

Each switch is explained with a history, images/videos. 

https://www.calling315.com/switches/keith-line-switchExplainer of the Keith Line Switch - operations and pictures with videos. Keith was the first to design a "line concentrator" for improving exchange switching efficiency. It was the forerunner of the Line Finder.

https://www.calling315.com/switches/strowgers-first-exchangeAlmon Strowger built the world's first automatic telephone exchange in La Porte Indiana in 1892. This page has images and history of the automatic exchange. The La Porte office was an "Exchange Lab" and had 5 different systems in a period of a few years.

https://www.calling315.com/sxs-exchangeThe step-by-step (SxS) telephone exchange is explained with images and videos.

https://www.calling315.com/telharmoniumReview of the Telharmonium streaming music machine. Cahill was the inventor of the first electronic music symthesizer using electromechanical parts. It shares much in common with telephone exchange technology so is included on this site. 

https://www.calling315.com/testing-systemsElectromechanical telephone exchanges needed large maintenance systems for repairing and testing the switches and relay-based components. This page explains how they work with examples and images for panel and crossbar exchanges.

https://www.calling315.com/traffic-engineeringThis section reviews Traffic Engineering principles for vintage telephone exchanges. It covers how Erlang, Poisson and Molina developed equations to assist traffic engineers to design cost effective systems. It reviews the key metrics and provides examples with many diagrams and charts. 

https://www.calling315.com/trunking-plansCentral offices were tied together using trunk lines. This enabled a connected network of smaller telephone exchanges to create the PSTN 

https://www.calling315.com/tribute-device-exploredThe Relay Tribute demo system is expanded upon with closeup pictures. 

https://www.calling315.com/underthehood2An overview of how the 8-line automatic telephone system works, with many pictures and explanations. It was a DIY STEM project. 

https://www.calling315.com/wiring-framingThis section explores the wiring and structural framing of vintage electro-mechanical telephone exchanges. Learn about wiring measured in "miles per foot" and some metrics for installing a panel system. 

 

https://www.calling315.com/zither-switchAlexander Keith invented the Zither or Piano Wire telephone switch. It could support 100 telephones. This page explains how it works and discusses the pros and cons of the device. This switch was a forerunner of an improved Strowger switch.

 Ref:TOC-calling315-meta_description_all.xlsx  

bottom of page