Where the offices are comparatively small, the incoming trunk portions of the B-boards are usually merely a continuance of the A-boards, the subscriber's multiple being continuous with and differing in no respect from that on the A-sections. Instead of the usual pairs of A-operators' plugs, cords, and supervisory equipment, there are on the key and plug shelves of these B-sections the incoming trunk plugs and their associated equipment.

In large offices it is customary to make the B-board entirely separate from the A-board, although the general characteristics of construction remain the same. The reason for separate A- and B-switchboards in large exchanges is to provide for independent growth of each without the growth of either interfering with the other.

A portion of an incoming trunk, or B-board, is shown in Fig. 378. The multiple is as usual, and, of course, there are no outgoing trunk jacks nor regular cord pairs. Instead the key and plug shelves are provided with the incoming-trunk plug equipments, thirty of these being about the usual quota assigned to each operator's position.

In multi-office exchanges, employing many central offices, such, for instance, as those in New York or Chicago, it is frequently found that nearly all of the calls that originate in one office are for subscribers whose lines terminate in some other office. In other words, the number of calls that have to be trunked to other offices is greatly in excess of the number of calls that may be handled through the multiple of the A-board in which they originate. It is not infrequent to have the percentage of trunked calls run as high as 75 per cent of the total number of calls originating in any one office, and in some of the offices in the larger cities this percentage runs higher than 90 per cent.

Fig. 378. Section of Trunk Switchboard
[View full size illustration.]

Fig. 379. Section of Partial Multiple Switchboard
[View full size illustration.]

This fact has brought up for consideration the problem as to whether, when the nature of the traffic is such that only a very small portion of the calls can be handled in the office where they originate, it is worth while to employ the multiple terminals for the subscribers' lines on the A-boards. In other words, if so great a proportion as 90 per cent of the calls have to be trunked any way, is it worth while to provide the great expense of a full multiple on all the sections of the A-board in order to make it possible to handle the remaining 10 per cent of the calls directly by the A-operators?

As a result of this consideration it has been generally conceded that where such a very great percentage of trunking was necessary, the full multiple of the subscribers' lines on each section was not warranted, and what is known as the partial multiple board has come into existence in large manual exchanges. In these the regular subscribers' multiple is entirely omitted from the A-board, all subscribers' calls being handled through outgoing trunk jacks connected by trunks to B-boards in the same as well as other offices. In these partial multiple A-boards, the answering jacks are multipled a few times, usually twice, so that calls on each line may be answered from more than one position. This multipling of answering jacks does not in any way take the place of the regular multipling in full multiple boards, since in no case are the calls completed through the multiple jacks. It is done merely for the purpose of contributing to team work between the operators.