The Panel Machine Switching System is a type of automatic telephone exchange for urban service that was used in the Bell System in the United States for seven decades. The first semi-mechanical types of this design were installed in 1915 in Newark, New Jersey, and the last were retired in the same city in 1983. The Panel switch was named for its tall panels which consisted of layered strips of terminals. Between each strip was placed an insulating layer, which kept each metal strip electrically isolated from the ones above and below. These terminals were arranged in banks, five of which occupied an average selector frame. Each bank contained 100 sets of terminals, for a total of 500 sets of terminals per frame. At the bottom, the frame had two electric motors to drive sixty selectors up and down by electromagnetically controlled clutches. As calls were completed through the system, selectors moved vertically over the sets of terminals until they reached the desired location, at which point the selector stopped its upward travel, and selections progressed to the next frame, until finally, the called subscriber's line was reached.
History In c. 1906, AT&T organized two research groups for solving the unique challenges in switching telephone traffic in the large urban centers in the Bell System. Large cities had a complex infrastructure of manual switching that prevented complete ad hoc conversion to mechanical switching, but more favorable economics was anticipated from conversion to mechanical operation. No satisfactory methods existed for interconnecting manual systems with machines for switching. The two groups at the Western Electric Laboratories focused on different technologies, using competitive development to stimulate invention and increase product quality, a concept that had been successful at AT&T previously in transmitter design. One group continued existing work that yielded the Rotary system, while the second group developed a system that was based on linear movement of switch components, which became known as the panel bank. As work continued, many subassemblies were shared, and the two switches only distinguished themselves in the switching mechanisms. By 1910, the design of the Rotary system had progressed farther and internal trials employed it at Western Electric as a private branch exchange (PBX). However, by 1912, the company had decided that the panel system showed better promise to solve the large-city problem, and delegated the use of the Rotary system for use in Europe to satisfy the growing demand and competition from other vendors there, under the management and manufacture by the International Western Electric Company in Belgium. After a trial installation as a PBX within Western Electric in 1913, Panel system planning commenced with design and construction of field trial central offices using a semi-mechanical method of switching, in which subscribers still used telephones without a dial, and operators answered calls and keyed the destination telephone number into the panel switch, which then completed the call automatically.
These first panel-type exchanges were placed in service in Newark, New Jersey, on January 16, 1915 at the Mulberry central office serving 3640 subscribers, and on June 12 in the Waverly central office, which had 6480 lines. Panel development continued throughout the rest of the 1910s and in the 1920s in the United States. A third system in Newark (Branch Brook) followed in April 1917 for testing automatic call distribution. The first fully machine-switching Panel systems using common control principles were the Douglas and Tyler exchanges in Omaha, Nebraska, completed in December 1921. Subscribers were issued new telephones with dials, that permitted the subscriber to place local calls without operator assistance. This installation was followed by the first installations in the eastern region in the Sherwood and Syracuse-2 central offices in Paterson, New Jersey, in May and July 1922, respectively. The storied Pennsylvania exchange in New York City was cut-over to service in October 1922. Most Panel installations were replaced by modern systems during the 1970s. The last Panel switch, located in the Bigelow central office in Newark, was decommissioned by 1983.
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