TELEPHONE (see 26.547). Various improvements were made during 1910-21 in the mechanism and working of the telephone, apart from the introduction of wireless telephony, which is dealt with in the separate article under Wireless. Since operating practice and the application of recent inventions are not always the same in the United States as in Great Britain, some developments which are not common to both countries are described in the section headed United States. At the end of that section will be found a table showing the expansion of telephone facilities during the years 1910-21 in all countries.
Great Britain
Private Branch Exchanges.—The increasing use of the telephone as between one party and another was early followed by a demand for the means of connecting different members of a firm in the same building without the necessity for providing each with a separate line to the public exchange. A further development required that incoming calls for a large firm should be dealt with at a central point in the initial stage so that after ascertaining the business of the caller he could be placed in communication with the particular department concerned. To meet these needs switchboards similar in general principles to those provided for main exchanges have come into use. The call from a local extension or from the main exchange is answered by the branch exchange operator, and the connexion is completed by the medium of connecting cords, or, on small systems, by circuits with which each line is associated by means of a key. In the latter case the depression of any pair of keys on a common connecting circuit places the two corresponding lines into connexion with one another. On the smaller installations where it is probable that an operator is not always available to give prompt attention to calling and clearing signals, the extension stations signal the completion of their conversation direct to the main exchange and simultaneously to the branch exchange. This enables the main exchange to disconnect the circuit promptly, releasing the main exchange circuit and any junction circuits which may have been occupied by the connexion. The prompt release of these circuits is of extreme importance in the economical working of the whole system. On larger branch exchange systems where an operator is in continuous attendance, the main exchange clearing signals are controlled by the withdrawal of the cord connexion at the branch exchange. The advantage to be gained under this scheme lies in the fact that the extension station can call in the branch exchange operator during a conversation and get an established call from the main exchange transferred to another extension station when required. When a branch exchange is used in conjunction with a common battery or automatic switching system, the current for speaking purposes is fed over the main exchange lines on exchange to extension station connexions, and by means of a power lead from the main exchange in the case of extension to extension connexions.
When associated with automatic switching systems the branch exchange operator is provided with a calling dial, so that on each exchange connexion she can dial the number required by any extension station.
Trunk Line Working.—Trunk or long distance working is complicated by the necessity for recording the particulars of all calls, and because instantaneous connexion cannot always be effected owing to the prohibitive cost of providing lines with such liberality as would ensure a no-delay service at all times.
The system of the British Post Office is worked as follows: A subscriber desiring a trunk connexion calls up his local exchange and notifies his requirements. If circuits are available to the town required on such a basis as to afford a no-delay service, the connexion is effected at once and the signalling and control arrangements are similar to those described for junction working. The operator records the particulars of the call on a ticket which is used for future accounting purposes. Should a no-delay service not be available, the operator records particulars of the requirements herself, or, in cases where lines to the town required terminate at a separate trunk exchange, she extends the subscriber's circuit to the trunk exchange and obtains a direct connexion to a special record operator whose sole business it is to note the particulars of the required trunk connexion. The subscriber is informed that he will be called later and the connexion is then severed. Meanwhile, the ticket is conveyed to the switchboard position where the lines to the town wanted are terminated. Calls at this point are dealt with in order of priority as recorded by the time on the ticket when the demand was initiated. Particulars of the connexion wanted are passed to the distant operator, who extends the circuit direct to the line of the “wanted” subscriber, when such lines terminate in the same exchange, or extends the circuit to a junction, when the “wanted”; subscriber is connected to another local exchange, and requests the operator at that exchange to effect the connexion. The trunk operator at the originating town simultaneously effects the connexion direct, or where another local exchange is concerned, by the medium of a junction, to the initiating subscriber, and when both subscribers are on the line, she completes the connexion. The call is controlled by the trunk operators, the junction circuit being equipped in such a manner that the subscriber's signals appear at the trunk exchanges, from which point disconnecting signals are sent automatically to the local exchanges when the connexions between the trunk and the junction circuits are removed.
Trunk exchanges are equipped with relays and lamps for! signalling purposes. “Calculagraphs” are employed for stamping the time of commencement and completion of conversation on the tickets. There is also associated with each trunk connexion a device which lights a lamp as soon as the scheduled limit of the period of conversation is reached.
Where the volume of traffic over any route is considerable, the requirements to the distant town may be notified over a separate circuit reserved for the purpose, and the local connexions involving the use of junction circuits to other local exchanges can be established in advance, thus minimizing the time of occupation of the main circuits and securing the greatest possible effective use of the trunk lines.
Manual Exchanges.—The main features of the manual telephone system remained in 1921 what they were in 1910. Improvements had been introduced to some extent, to diminish or eliminate altogether portions of the operator's work, but common battery transmission and signalling, connecting by means of plugs, flexible cord conductors and jacks, lamp calling and supervisory signals—all these remained unaltered.
Among the improvements referred to may be mentioned: Keyless ringing; automatic listening; secret service; ringing tone; automatic ringing cut-off; traffic distribution; ancillary answering jacks.
1. Keyless Ringing is the feature of a cord circuit which provides that the ringing of the required subscriber's bell commences automatically on the operator connecting to his line and ceases automatically on the removal of the receiver from its rest by this subscriber. This renders unnecessary the provision of a key for ringing purposes—hence the term “keyless” ringing.
2. Automatic Listening is a feature which eliminates the listening key. The operator's telephone is automatically connected to the calling subscriber's line when the answering plug of the cord circuit is inserted in the answering jack of that line. The operator's telephone is later automatically disconnected when, after ascertaining the number required by the calling subscriber, the operator connects the calling plug to the line of the required subscriber.
3. Secret Service follows from 2. Automatic listening involves the feature that while conditions suitable for conversation are established the operator's telephone is disconnected and without the aid of a listening key it is impossible for the operator to listen to a conversation.
4. Ringing Tone is a tone (distinctive from that intimating to a calling subscriber that the line he requires is engaged) applied to the calling subscriber's line while the bell of the called subscriber is being rung. Hearing this tone, which intimates that the bell of the called subscriber is being rung, and receiving no reply after a reasonable period, the calling subscriber infers that his correspondent is not available and restores his receiver. This facility reduces the time spent on “no reply” calls, by both operator and subscriber.
5. Automatic Ringing Cut-Off provides that the ringing of the required subscriber's bell automatically ceases when the calling subscriber, receiving no reply, decides to abandon the call and restores his receiver to its rest.
