Director telephone system
A subscriber in Wimbledon, for example, would be assigned the number WIMbledon1234; the first three letters, written in capitals, indicated the exchange code to be dialled.
The exchange code digits dialled by the calling subscriber were the same from any telephone in the London director area, which has a linked numbering scheme.
If written merely in capitals it indicated that the desired number was on an exchange which had not yet been converted to automatic working, and that the caller should dial only the initial three code digits, and wait to be connected by an operator.
Later some of the remaining manual exchanges were equipped with Coded-Call Indicators (CCI) which displayed the local digits dialled by the caller to the operator, The number would be listed as for an automatic subscriber with the first three letters in bold, and automatic subscribers would dial all seven digits.
Special Service numbers apart from “0” for operator used a three-letter exchange name; e.g. TOL (toll), TRU (trunk), DIR (directory enquiries), TIM (time, the “speaking clock”), ENG (engineering i.e. faults), or UMP (for the Test Match cricket scores).
The A-digit selector returns dial tone, steps to the first dialled digit and searches for a free director from that group.
This selector has six banks to permit up to six pulse trains to be generated by the director to step the code selectors; the bank contacts are strapped via a translation field to indicate the digit required (or to a DCO lead to indicate that all translation digits have been sent).
This is generated under the control of the send switch uniselector as it searches for the marks provided by the translation field straps for the exchange code received.
Seizure of one of these directors via level "0" passes a discrimination signal to it which causes it to pulse out the routing digits to give access to the operator with no further dialling required by the subscriber.
The busy routes should also be allocated to the lower levels on the selectors to minimise setting-up time and also the number of operations of the mechanism and hence the wear on them.
[10] Before a manual exchange was converted to automatic, changes to allocated numbers to facilitate PBX hunting groups were required.
[11] Holborn exchange had 9,400 subscribers (lines) and had 222 directors, each of which on average handled 72 calls in the busy hour and be held for 20 seconds.
However the director system had the advantage of using equipment items similar to the SXS exchanges being used in smaller British cities and towns, and would be manufactured in Britain from the outset.
This problem was sidestepped to some degree by using otherwise unusable codes to give access to the fringe non-director exchanges from the director area.
Generally it was policy to install director equipment where the total number of subscribers was expected to be greater than 60,000 lines within 30 years.
[17] In cities outside London, it was possible to avoid the installation of Coded-Call Indicator (CCI) equipment at manual exchanges by converting to automatic in stages with the issue of a new directory, e.g. converting groups of (say) six exchanges at 12-month intervals; this was done in Liverpool, Birmingham, Glasgow and Edinburgh.
[19] By the time the director system was superseded in the UK by all-figure dialling from March 1966, London had almost exhausted all possible, usable combinations of letters (it had about 240 at that stage).
Los Angeles being a small town early in the 20th century and partly being served by an independent telephone company, grew to be a major exception.
Before the advent of electronic switching systems, directors were commonly used in areas of the city served by GTE.
