Panel switch

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. 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 focussed on different technologies, using a competitive development approach 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.