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The Panel Telephone Exchange

This section outlines the process of making a call using a panel exchange. Please note that this content is still under development. Wait, don't go....

Interestingly, the 7A rotary exchange shares many operational characteristics with the panel exchange, making them closely related. They were both developed by Bell System engineers starting circa 1906. The two design groups were in a sort of competition to build the world's first metro-class exchange. Both the panel and 7A rotary designs gained prominence and incorporated similar technologies. While the panel was produced and promoted in North America, the 7A rotary, initially made in Belgium, was marketed globally except in North America.    

                                 Fig 1, Panel dial system elements (1922)

Explainer, Panel dial system elements in switching chain. Line finders, selectors, sender

Understanding the 7A rotary system provides insights into the panel system because they share many parallel concepts. Sure, the switches are completely different in style but ideas of call progress are similar. See the links below. They contain explainer videos too.

  7A rotary exchange (Note: Panel Sender in Fig 1 is functionally similar to 7A Register) 
  The panel switch (line finder and selectors switches in Fig 1)
  Sequence switch (associated with each selector switch in Fig 1) 

The Office Selector switch (Fig 1) is not present in some smaller exchanges. Think of this switch as a way to extend the District Selector in exchanges that connect to many remote offices. 

Video demo

To get a feel for both the line finder and selector switches in action, view the making a 7-digit panel call. There is annotation but no dialog. The line finder switch is activated before any digits are dialed; it is the first switch shown in the video. The selector switches follow and move based on the dialed digits.

First the office code 722 is dialed. The District Selector is directed to connect to an Incoming Selector and then the digits 7+2+3+4 are dialed and advance the Incoming and Final selectors respectively. Of course, there are control means but these are not shown or explained in this short video. 

For further exploration, the Connections Museum of Seattle offers videos and other content related to the panel exchange.  See for example, panel exchange walk around, no narration.

A planned narrated and animated explainer video is under construction. It will follow the dialing and show the steps required to complete an intraoffice call in a panel system. 

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