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Connolly and McTighe Switch

Highlights

Switch category: Rotary


 

Inventors: M. D., & T. A. Connolly & T. J. McTighe (Philadelphia, US)
 

Important dates: US patent 222,458, December 9, 1879. The Connolly system was exhibited at the Electrical Exhibition in Paris in 1881 [Kingsbury].  
 

Legacy: A first -- no operator needed. Subscriber dials the desired number. A rotary switch at the Central Office responds to the dialing and a talking path is established. Only two wires (or one and ground) to a subscriber's set from the CO.

Background

In 1879, three years after Alexander Graham Bell's invention, the first patent was filed relating to an automated switching and apparatus.

Despite its crudeness, this first system embodied the basic principle of future dial systems.  At each subscriber station, in addition to the telephone, battery, and call bell, were a reversing key, a compound switch, and a non-rotary dial. The multiple steps required to make a call could easily confuse users.

Switch details

This system uses a finger-turned dial at the subscriber’s telephone. Turning it sends to the CO a series of current interruptions based on the desired number.  At the CO, the switch uses an electromagnet attached to a pawl and ratchet to move a wiper, one step at a time, stopping at the called number’s terminal. See Fig 1. There is only one moving wiper. This is sufficient to create the talking path in this design. 

Most authors credit Froment (in France) almost 30 years prior with a similar idea but for transmitting a telegraph code (A-Z) using labeled sending and receiving wheels.

The novelty of Connolly’s invention begins with the duplication of the switch’s ratchet wheels E; there is one for each subscriber and each ratchet has an attached wiper.  

Connolly and Mctighe telephone switch image from patent

Fig 1, dial (a) and (b) CO switch per-subscriber from [Hill]

In Fig 2, the caller #9 dials #3. Note the marked positions of the dial and the switch ratchet are a 3. The caller dial is rotated 3 steps, the CO switch ratchets to position #3 and a talking connection is made using the single movable wiper contact.   

 

Using power from the subscriber battery to ratchet the electromagnet is impractical. Arthur Bessey Smith [Smith] outlines several issues with this design. Connolly was granted other patents to fix some of the main objections, but no fix would be sufficient for the design to go mainstream.

     

Connolly and McTighe patent figure for telephone switch

                                    Fig 2, from patent, subscriber #9 calls #3

 
Fig 3 pictures a 6-subscriber switch from 1880. Expanding this to 30 users would make it 5x larger.  The monolithic design poses challenges for expansion. Effective exchange designs tackle the switching problem using a divide and conquer approach. One large switch is usually a bad way to go for scaling purposes.

Connolly and McTighe 6 line telephone switch image 1880

           Fig 3, 6-subscriber Connolly and McTighe switch, 1880  [Smithsonian]

 

References

Hill, R.B., Dial telephone systems, Bell Laboratories Record, Jan 1953

 

Kingsbury, j. e., Telephone and telephone exchanges; Their invention and development; Longmans, Green, and Co, 1915  

 

Smithsonian, American History Museum

Smith, Arthur Bessey, The Early History of the Automatic Telephone, circa 1909

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