As every line appeared at several points, so as to be accessible to every operator, it be came necessary to provide some means for in dicating to the operator desiring to complete a connection, whether or not the called line was already in use. To accomplish this Charles E. Scribner, for many years chief engineer of the Western Electric Company— the manufactur ing department of the Bell Telephone System —invented the click busy test. The circuits were so arranged by him that when one opera tor had inserted a plug in the jack of any line, an electrical potential was placed on the rings of all of the jacks of that line which were multipled before the operators. Any operator desiring to complete a connection with a line was required to touch the tip of the connecting plug against the ring of the jack with which connection was desired before completely in serting the plug into the jack. If the desired line was already in use, the potential on the ring of its jack would cause the operator to hear a "click" in her head telephone, thus in dicating that the line was already in use through a connection established at some other part of the switchboard. These two very im portant principles— the multiple and the busy test—have persisted, in one form or another, in every large switchboard up to the present time. A serious objection to the series type of board was that dirt or dust, accidentally lodg ing in the spring jack contacts, would introduce high resistance into the line, or even cause the circuit to become open, thus interfering with or quite preventing conversation over the line. Another objection to the series board was that the circuit in the switchboard was unbalanced. To overcome these defects Scribner, applying Carty's bridging principle to switchboards, de vised the "branch terminal" multiple board. (Fig. 41)). With this arrangement of circuits, two wire system, the switchboard had to be adapted throughout to the new plan of working. This meant a wholesale displacement of exist ing grounded line switchboards by boards of the newer metallic circuit type.
For a long time after multiple switchboards came into use, batteries were required at each telephone station to furnish the current for actuating the transmitter, and magneto-genera tors, operated by turning a crank, were em ployed for signaling the central office when ever a connection was desired. The generator was also used for signalling "disconnect," and was not wholly satisfactory for this purpose, as subscribers frequently neglected to give the dis connect signal, making it necessary for opera tors to "listen in" on connections and ask RAre you through?" The invention and development of the common battery system did away with the batteries and magneto-generators at the telephone stations. With this system the only battery required was a large centralized or common storage battery at the central office. This system, furthermore, did away with all mechanical line and disconnect signals in the bridged multiple taps were provided from each side of the line to springs in the jacks in the multiple before the operators, and the line sig nals were wound to a high impedance. By
doing this, and adding a locking winding to prevent false operation, the need for disconnect ing the line signals from the circuit when the line was in use was avoided. A feature added at this time was the uself-restoringo drop (line signal) which provided for the automatic re storing of the annunciator to its original posi tion upon the insertion of a plug in the answer ing jack of the calling line. Before this time, the annunciators had to be restored by the operators by hand.
The metallic circuit principle (described be low) was introduced in switchboards at an early date, for as soon as any of the lines in a telephone office were changed to the metallic, or switchboard by enabling tiny electric lamps, governed by relays, to be substituted for them. This was one of the most impprtant improve ments ever made in telephone switchboards. The subscriber, connected with a common bat tery board, calls the central office by merely re moving his receiver from the hook, the motion of the latter causing a lamp associated with his line to light before the operator. Com pletion of the conversation is indicated to the operator by suitable lamp signals in the cord circuit, when the receiver is replaced upon its hook. The circuits of common battery switch boards are of two types,— the impedance coil type and the repeating coil type. Fig. 41 shows a typical circuit diagram of the latter type, which, is the more generally employed. Fig. 42-A shows the front view of a modern com mon-battery switchboard. The operators sit on chairs in front of the switchboard. Fig. 42-B shows the detail of a single position. Where the telephone message is the unit of service charge, each subscriber's line is provided, at the central • office, with an accurate message registering device, by means of which each completed con nection can be recorded.
Trunking Systems.— In large cities it is impracticable to concentrate all subscribers' lines at one central office on account of the dis tances involved, which affect both the resistance of the lines and their cost, and also on account of the fact that more than one switchboard would generally be required. Where several central offices are required in one city, connec tions from subscribers whose lines terminate at one office to subscribers whose lines terminate at another office must be completed with the aid of trunk lines connecting the offices. In the early days, an operator, initiating a trunked connection, by applying the busy test, selected an idle trunk line over which she called the operator at the distant office by ring ing as though the trunk were a subscriber's line. When, in response to a signal operated by the ringing current, the operator at the distant office answered, the first-mentioned operator transmitted to her the number of the line with which connection was desired and this line was then connected by her to the trunk line.