Trunking was greatly improved by the ad vent of the reverse call-wire method. With this method the operator at the office where the call originates (known as the "A" operator), by depressing a push button key, of which she has one for each central office in the district, establishes connection with a call circuit per manently connected with the head telephone of the operator at the distant office (known as the °B" operator). The number of the de sired line is then transmitted over the call cir cuit to the °IV operator, who, by visual in spection, selects an idle trunk line and trans mits back its number to the °A" operator, who at once connects the calling party with that trunk line, the °IP operator meanwhile estab lishing connection between the other end of the same trunk line and the line of the called party. The call circuits are used-for no other purpose than communication between operators.
Trunk circuits have been designed, and are in general use, which allow the "A" operator to supervise a connection completed in a distant office as readily as would be the case with a connection completed in her own office. The same is true of machine ringing circuits whereby the bell of the called subscriber at the distant office begins to ring automatically as soon as the trunk line plug is inserted in the jack of his line without the °B" operator being required to operate a ringing key.
Toll Boards.— The term °toll connections is applied to connections between subscribers so far removed telephonically from each other that a special toll charge is made for the con nection. On account of the fact that the toll lines between one place and another have to be grouped together for efficient operation and that each toll message has to be carefully timed and a ticket record made of the call, toll con nections generally require special switchboard circuits and apparatus at the central office and the service of special operators. In the case of the smaller central offices the toll switchboard is sometimes combined with a portion of the local switchboard. In larger places, to enable adequate toll service to be given, separate large and complicated toll switchboards are ordinarily required. These toll switchboards are usually centralized at one point in the city and are connected by special trunk lines with the local switchboards in the several central offices of the city. In cities of the largest size more than one toll switchboard centre may be required.
Party Lines.— Serving more than one tele phone user by means of a single line (party line) enables those having but little use for the telephone to obtain service on a basis which otherwise would be impracticable. The suc cessful operation of the party line depends upon the Carty bridging bell principle already de scribed. The extensive use of party lines to
serve farmer's or rural stations, by destroying the isolation of the farmer,. has produced a wonderful improvement in Ms economic and social life by placing him in constant touch with markets and sources of supply, and with the weather bureau, and has provided for his family a means of communication available not only for social purposes, but also ready at all times for summoning help in the event of sick ness or danger. On party lines equipped with two stations,•and on some equipped with four stations, means have been devised whereby the bell at a given party line station may be rung, the others meanwhile remaining silent.
Private Branch Exchanges.— Important adjuncts to the telephone central office are the private branch exchanges. These consist of switchboards, generally of small size, located in such places as hotels, apartment buildings or large business houses and having connected to them all the subscriber's lines in such estab lishments. Private branch exchanges are also connected with the nearest telephone central office by a sufficient number of trunk lines to handle the traffic arising from and flowing to them. Calls originating within the establishment where the private branch exchange is located, for parties within the same establishment, are completed by the private branch exchange operator without passing through the telephone central office. The introduction of the common battery system greatly facilitated the operation of private branch exchanges and enabled the necessary talking current to be supplied over trunk lines from the central office. Fig. 43 shows a typical private branch exchange of moderate size. Some of the largest private branch exchange switchboards are comparable in size to central office switchboards serving large communities.
Automatic Systems.— Throughout the de velopment of the manually operated switch board, the tendency was continually toward in creasing the use of automatic labor-saving ma chinery. In the most modern forms of manually operated boards a complete analysis of all the operations involved shows that a large proportion of them are performed auto matically. The term ((automatic," or switching,° is, however, applied to systems in which the number of operators required at the central office is reduced to a relatively small number, the switching functions otherwise per formed by operators being, to a greater or less extent, performed by electromechanical ap pliances. During recent years there has been extensive development of machine switching systems. The apparatus is highly complex and ingeniously devised.