Perfluoroalkyl Acids: What Is the Evidence Telling Us? Volume 115, Number 5, May 2007. Evaluation of perfluorooctane 3rd geneva convention pdf in a wastewater treatment system and in a commercial surface protection product”.
PFOS-PFOA Information: What is 3M Doing? This page was last edited on 8 December 2017, at 04:25. It identified the switching system to which a telephone was connected. Areas or cities with more subscribers were served by multiple central offices, possibly hosted in the same building. The leading letters of a central office name were used as the leading components of the telephone number representation, so that each telephone number in an area was unique.
These letters were mapped to digits, which was indicated visibly on a dial telephone. T proposed a mapping system that displayed three letters each with the digits 2 through 9 on the dial. Telephone directories or other telephone number displays, such as in advertising, typically listed the telephone number showing the significant letters of the central office name in bold capital letters, followed by the digits that identified the subscriber line. Telephone exchange names were used in many countries, but were phased out for numeric systems by the 1960s.
Telephone numbers listed in 1920 in New York City having three-letter exchange prefixes. This system mapped the letter of the telephone number to the digits on the telephone dial. Most other major Canadian and US cities, such as Toronto and Atlanta, were converted from manual exchanges using four digits to a local 2L-4N numbering plan. In small towns with a single central office, local calls typically required dialing only four or five-digits at most, without using named exchanges. A toll call required the assistance of an operator, who asked for the name of the town and the local station number. Mulberry and Waverly exchanges in Newark, New Jersey. When the technology first appeared in the Mulberry exchange, subscribers had no dials on their telephones and the new system was transparent to them — they asked an operator to ring their called party as usual.
However, the operator keyed the number into the panel equipment, instead of making cord connections manually. By the 1950s twenty cities were served by this type of office. From the time of these first conversions to automated equipment in the 1920s, through the conversions of most manual equipment by the 1960s, it was necessary for telephone numbers to be represented uniformly across the nation. By Bell System policy, customers never needed to be concerned about whether they were calling an automatic or a manual exchange. The 2L-5N system became the North American standard, as customer-dialed long distance service came into use in the 1950s. North American 3L-4N example, used in only large cities before conversion to two-letter central office names.
This format was in use from the 1920s through the 1950s, and was phased out by ca. 2L-5N format, gradually implemented continent-wide starting in the 1940s, in preparation for DDD. The H in Hill, although not dialed, is still capitalized as the first letter of the second word. TEmpleton 1-6400 would appear as TE 1-6400. If the central office was known by a name, but no letters were dialed, it was common to capitalize only the first letter of the central office, e. Such numbers were assigned typically in manual offices, and the name would be spoken by a subscriber when requesting a destination.
Often these were geographically significant names, such as the town’s name. In large cities with coexisting manual and dial areas, the numbering was generally standardized to one format. For example, when the last manual exchange in San Francisco was converted to dial in 1953, the numbers had for several years been in the format of JUniper 6-5833. JUniper 4 was automatic, but JUniper 6 was manual. To call JUniper 6 from JUniper 4, the subscriber dialed the number and it was displayed to the B-board operator at JUniper 6, and that operator would complete the connection manually.