Panel switch

The Panel Machine Switching System is an early type of automatic telephone exchange for urban service, introduced in the Bell System in the 1920s. It was developed by Western Electric Labs, the forerunner of Bell Labs, in the U.S., in parallel with the Rotary system at International Western Electric in Belgium before World War I, which was used in Europe. Both systems had many features in common.

The first Panel-type exchanges were placed in service in Newark, New Jersey, on January 16, 1915 at the Mulberry central office, and on June 12 in the Waverly central office. These system were semi-mechanical systems using telephones at customer stations without a dial. Operators answered calls and keyed the station number into the panel switch. The first fully machine-switching Panel systems using common control principles were placed in service in Omaha, Nebraska in December 1921, followed by the PEnnsylvania exchange in New York City in October 1922. Most Panel installations were replaced by modern systems during the 1970s and the last Panel switch was decommissioned in the Bigelow central office in Newark by 1983.

The Panel switch was named for its tall panels covered with 500 rows of terminals. Each panel had an electric motor to drive usually sixty selectors by electromagnetically controlled clutches. The selector was similar in effect to a stepping switch though it moved continuously over the contacts. Each selector had five brushes, each of which could select from 100 terminals arranged in groups. Pulses were sent in the opposite direction than in the preexisting Strowger switches, i.e. from the selector to the register which had received the dialed digits from the caller, a signaling method that became known as revertive pulsing.

As in the Strowger (SXS) system, each central office could address up to 10,000 numbered lines, requiring four digits for each subscriber station.

The panel system was designed to interconnect the offices of a city or a local calling area. Each office was assigned a two- (later three) digit office code. Callers dialed the office code followed by the station number. In most situations this led to six-digit (later seven) numbers. But from the beginning the panel system handled seven-digit numbers (later eight), for two reasons. Party line numbers were listed with one of the letters J, M, R, and W following the line number. The caller dialed the office code, the line number, and the digit corresponding to the letter. The panel system was designed to work with manual offices of up to 10,500 lines. To call a line in a manual office, callers dialed the office code followed by the line number, just as they would when calling a number in a fully automatic office. For lines 10,000 and up, callers therefore dialed the office code and a five-digit line number.

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