The 500 switch – a workhorse

Since the mid-1920s, calls from countless subscribers around the world with dial phones have been connected by 500-switches in telephone stations from Ericsson. The first station with the 500-switching system in regular operation was opened in 1923. Eight years later, Ericsson had delivered about 100 stations with a total of more than 350,000 lines. Sales continued to increase over several decades and did not decline significantly until the 1970s. By 1974, 4.8 million lines using this system were in operation in public telephone stations.

The development of the 500-switching system, with its characteristic selector bars traveling across bare wires, has a somewhat tortuous history. The initial idea for the system was conceived by Axel Hultman, director of Televerket, the Swedish PTT, in Stockholm.

After conducting experiments at the public telegraph service over a period of several years in the area of automatic switching, Hultman, acting as a private person, signed a cooperative agreement with Ericsson in the spring of 1913. While Ericsson had made considerable progress in the development of manual switching technology, the company was losing ground to competitors in automatic telephony.

According to the cooperative agreement, the parties would develop Hultman's ideas and apply for patents in his name. The contract granted Ericsson the right to use future patents, while Hultman was to receive royalty payments for the company's sales of switching equipment based on his patents. Ericsson's right of use, however, was limited in Sweden to deliveries to Televerket, thus effectively excluding its primary competitor in Stockholm at that time, AB Stockholmstelefon, which was a subsidiary of Stockholms Allmänna Telefonaktiebolag (SAT).

During the following years, development work was pursued on a system with 10,000 line switches that traveled over bare wires. In addition to Hultman, Ericsson engineers Martin Löfgren and Sigurd Johanson participated in this work. The 500-system's switches, which selected among 500 lines, first in a rotary, and then in a radial movement, was created after the merger of Ericsson and SAT in 1918 and was the result of a parallel development project started by Ericsson and led by the company's engineer Knut Kåell.

It is interesting to note that Löfgren had experimented with a design using a similar switch movement prior to 1918 but that he had abandoned that effort, since it would have infringed on a patent held by SAT. This fact, in combination with Ericsson's limited right to use Hultman's patent, led Löfgren to believe that the design could not possibly be used in Sweden and that the primary reason was the fierce competition in telephony between Stockholmstelefon and Televerket.

The merger between SAT and Ericsson in 1918, however, eliminated the competitive situation, which in turn influenced further development of the 500-system.

A number of parallel disputes characterized the following period. Martin Löfgren, supported by others within Ericsson, claimed that he had strongly contributed to equipment designs that had subsequently been patented in Hultgren's name. Management, however, showed little understanding for Löfgren's arguments. Another dispute concerned the decision process then in progress within Televerket with respect to the choice of an automatic switching system for its Stockholm network, which had also been expanded significantly through the acquisition of Stockholmtelefon's network.

The task of merging the two networks fell to none other than Axel Hultman. With the purchase of the competitor's network, he had become Televerket's head engineer in Stockholm. Hultman was critical of the 500-switch design in several respects, despite the fact that it was based on his own patents, preferring instead a design with larger switches. Included in the procurement process being conducted by the public operator were a number of firmly established foreign systems, as well as a Swedish system developed by G A Betulander and Nils Palmgren, that had all performed favorably in testing.

When Televerket finally selected Ericsson's 500-switch system in 1921, this marked the end of a protracted decision process in which both Ericsson and Televerket had been forced to make compromises. Furthermore, the crossbar switching system developed by Betulander and Palmgren had virtually been eliminated, in part through Ericsson gaining control over Betulander's company in December 1919, and in part because Hultman had successfully argued that the crossbar system was less suitable for large switching stations.

The first 500-switch stations, which were placed in operation in Rotterdam in 1923, followed by Stockholm the year after, were thus the product of extensive engineering science. The many stations successfully taken into operation over the years underscore the system's functional capacity, particularly considering the fact that there were competing systems in many cases that were developed by companies that had considerably greater experience of automatic telephone systems. This should not obscure the fact, however, that this was engineering science in the broadest sense, which included both scheming and bartering with respect to design choices, and that shaped and created the workhorse that the 500-switch system was to become.

Author: Claes-Fredrik Helgesson

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2 comments
  • Youtube video of LME 500 selector

    by David Crozier

    We have a simulated (motor driven) short video of a 500 line switch going through a routine at
    www.youtube.com/watch?v=RCXJ-Px_JDo