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Lorimer Switch

 

Basic cylindrical switch design
 

The objective of the switch design was to establish a talking-path between exchange subscribers based on a “dialed number.” 

The Lorimer switch was motor driven, had 360 degree rotating wipers and a cylindrical terminal bank. Its development was likely influenced by Romain Callender's work (Appendix A). It shared little in common with Strowger's famous 2-dimensional "linear/rotary" switch from 1890.

 

To get a sense of the appearance of the switches installed in an exchange, see the three pictures below, a composite Fig 1. Image 1: 500-line exchange with cylindrical switches. 36 individual switches can be seen in the front rack. Image 2: System driven by a common motor with a stabilizing fly wheel. Image 3: An individual disassembled switch.

 

In Image 3 notice the core wiper assembly of 44 wipers (11x4). These rotate and make connections with the outer fixed contact banks. These figures are from [British] and [Magnus]

In Image 1 below, each row of switches performs a different function.  The switch mechanics are nearly identical, but there are some minor differences between them. On the top row are seven "line finders" (LF).  This is the first use of the idea of using a switch to "find lines" and connect calling subscribers to idle selectors to accept the dialed digits.

The row below the LF are seven selectors similar to the function of what a Strowger selector switch does. The overall design is complex and a testament to the Lorimer brothers ingenuity. See [Magnus] and [Smith] for a good coverage of exchange operations.   

Highlights

Switch category: Rotary (360 degrees)

Inventors: James, Hoyt, and George Lorimer (Canada). See Appendix A.

Important dates: US patent 1,187,634 filed 1900, first installation Peterboro, Ontario,1905; The British Post Office contracted for an evaluation exchange at Caterham ~1912. A 500-line exchange was eventually installed at Hereford UK in 1914; 1923 (bankrupt).

Legacy:  The significance of the system was recognized by AT&T (Western Electric) who had been struggling to develop a system of their own.  They bought the patent rights from the struggling Lorimer company in 1903 and used it as a basis for a reliable system of their own design. In time, AT&T developed the Panel and 7A Rotary systems using some key ideas from the Lorimer brothers. Especially notable is the first use of a the “Line Finder” switch and “revertive pulsing”. 

Lorimar telephone switch im Hereford UK 1914
lorimer telephone exchange showing motor drive and 35 switches
lorimer switch disassembled showing insides of the mechanism

Composite Figure 1 (3 images)

In the figure directly above the wipers have been removed from the center of the stationary cylinder.  Under operation, the wipers rotate continuously and stop via a clutch on the desired fixed terminals to make a talking connection. There are 12 rows and 44 terminals per row (528 contact points total/switch). Notice the wires attached on the cylinder's outside surface.  As discussed elsewhere, the switches are interconnected to create a much larger switch fabric to connect  the needed talking paths.  

Each switch unit drives its 360 degrees rotary wiper motion by a clutch engaging a motor driven shaft. Individual switches did not have dedicated power to move their contact wipers without the external motor. The wipers of each switch were in constant rotary motion unless disengaged from the shaft by a clutch, to make a temporary speaking connection.  

 

The common motor motif, but not the cylindrical switches, was used in Western Electric’s Panel and Rotary 7A exchanges and these achieved wide acceptance respectively. So, the Lorimer brothers left an influential legacy by virtue of their valuable first patent in 1901 and a nearly identical one assigned to Western Electric, US patent 1,294,285.

 

 

 

                                        

 

 

 

 

                                     Fig 2, Lorimer switch components [Magnus]

 

The brothers designed a new type of telephone to control the exchange switches. This was before the rotary dial was invented in 1898 by Alexander Keith. See Appendix B for more on their telephone and its unique dialing operations.  

Lorimer switch elements from the patent

Rotary switch internals   

 

The continuously rotating wiper assembly is quite sophisticated. Fig 3 and some text is from [Magnus]. It provides a good view of the wipers and the numbering arrangement. Each wiper assembly contains up to 12 wipers in a vertical column. There are four assemblies, offset by 90 degrees. In this way, the switch is divided into four quadrants.  

For the purposes of this explanation, the figure shows only 2 or 3 wipers per quadrant. Lines 01-99 occupy rows 1-3, lines 101-199 occupy rows 4-6 (not shown) and so on. It’s easy to see how one switch could support 400 subscribers (with a 1-wire talk path) with 12 rows of terminals. Of course, only one talking connection can be established at the same time per switch. Using the conventional 3-wire (T/R/S) talk path, a single switch can support up to 100 subscriber connections. 

The ten cylinder wipers, attached to the common gearing labeled 'S', are individually connected to the Register Switch bank's contacts using the ten labeled leads.  The register switch and wiper have their own gearing and moves independently of the cylinder wipers.

 

 

 

 

 

                                   Fig 3, Wipers and bank arrangement

 

To connect calling Line A to terminal #89, the Register Switch wiper is first moved to bank contact 80. The cylinder gearing S is independently rotated until the unit’s wipers are on the cylinder contact points having 9 as their unit’s digit. Line A is thus connected via 80, lead J and the #8X cylinder wiper to terminal #89.  

 

Using this type of switching, a Lorimer exchange design using line finders and selectors (Lorimer called these connectors) and, if needed, interconnectors, could support up to 10K lines according to their granted patent.

Lorimer switch, esplainer, wipers and bank layout

Final words

 

The choice of requiring a custom and expensive telephone dialing mechanism was a liability and likely killed its chance of becoming widely accepted.  The switch’s bearings, clutches and the motors needed regular attention so they could perform according to design. Plus, each switch was in near constant rotation. This too was a liability. 

 

See picture of a Lorimer telephone in the advertisement below. Its dial (“indicator plate”) has four slide switches to register the desired number to call.

Lorimer telephone exchange advertisement -ad- showing switches and custome dial phone

                                       Fig 4, Advertisement circa 1920’s [British]

 

 

References

British: https://www.britishtelephones.com/lorimer/description.htm 

Magnus McLean, "Modern Electrical Engineering - Volume VI", Gresham Publishing Company Ltd, London, circa 1920

 

Smith, Arthur Bessey, The Early History of the Automatic Telephone, circa 1907 

 

Telephonecollecting.org/Bobs%20phones/Pages/Lorimer/Lorimer%20Brothers.htm

  Appendix A

     Romaine Callender's Contributions

  

The Lorimer brother’s inventions did not spring from whole cloth. They had several years of experience working for Romaine Callender in related activities.

 

Romaine lived in Brantford Ontario, Canada, and was a music teacher. He invented an organ player and sold its patent.  He was a born inventor.

 

He was fascinated by the telephone field and began to develop methods and switches for an automatic exchange. He formed a workshop to develop his ideas and hired George and Hoyt Lorimer about 1892 as assistants.

 

Callender as lead thinker, and the Lorimer brothers as assistants, created several prototypes.  Despite their promise, these systems were ultimately unsuccessful, leading to significant loss of investor funds. Some of his concepts were genius but attaining consistent, reliable, performance evaded the team.

 

One notable concept from Callender is the 'Numeralizing' scheme, as illustrated below.

 

  

               Numeralizing methods from [Smith], partial redraw by the author

Romaine Callender switch  Numeralizing methods , partial redraw

There are 3 key components,

  • Tens and Units receivers

  • Circular “Selector-Connector”

  • Seizing electromagnet

 

The basic idea is to receive 2 digits from the calling telephone, and these set the Tens and Units numerical digit receivers respectively. Means for setting is not shown.

 

The circular switch contacts 0-4 represent the full ten from 0-9, also 10-14 represents 10-19, so that there will be a full 100 lines, forming a complete circle.

 

The arm F rotates continuously (motor driven) and passes the all the fixed contacts on the Selector-Connector. The rotation stops only when the “digits align.” Inspecting the figure, the Tens receiver is set to 2 and the Units is set to 1. So, #21 is the number to be called.

 

As arm F rotates, the Seizing electromagnet will engage when the bridging wipers G & H pass location 21 on the arc. This action causes arm F to stop rotation (clutch disengages) and a talking path can be made by other contacts not shown in the figure.

 

This arrangement looks suspiciously like the Lorimer brothers design (greatly improved) of several years later. Solving the “switching problem” involved financers, inventors, electrical specialists, and machinists over many years. Romaine Callender did his part to forward the state of the art.  [Smith] has a good working coverage of Callender’s efforts.

                                                              Appendix B

                                                     Dialing the Lorimer way 

By August 1897, the Lorimer brothers had built a crude prototype demonstrating a telephone controlling their rotary switches.  Their dialing method was complex and needed an expensive clockwork-like mechanism inside every phone. See Fig B1 [Magnus]  

Lorimer telephone with custom dial mechanism with hand crank

Fig B1, Lorimer telephone with dialing mechanism removed

Basically, a subscriber sets the four levers to the desired called number (ex, 1683), turns a crank one revolution to wind a spring, and goes off hook to automatically start the dialing process. Fig B2 ([Smith] modified by author) shows a simplified view of the dialing operation for one digit.

This method is the first example of revertive pulsing being applied to control an exchange switch. Here is an example of a dialing sequence assuming the subscriber’s unit’s digit lever is set to “3”.

 

  • In the figure, wipers A and B are ganged and part of the same, motor driven, rotary switch. The wipers continuously follow a circular path, one revolution every 2.5 seconds.

  • Wiper A, creates a pulse train and, acting over line L2, steps electromagnet 33 and this moves arm 142 over the station-side contacts.

  • After 3 revertive pulses, arm 142 strikes the pin grounded by brush G –digit 3

  • At once, relay 215 is energized and the rotation of the exchange switch stops and the station side electromagnet ceases stepping. The units digit 3 has been recorded on the exchange side switch.


The revertive pulses advanced a “counter” and its value corresponds to the position state of the exchange switch. This idea became widespread and was used with panel and rotary switch systems too. Albeit for these systems the counter means was moved to the exchange side and was not inside the subscriber’s telephone.

Dialing the Lorimer way: Interactions between telephone and central office switching

Fig B2, Interactions between telephone and central office switching 

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