Page:EB1922 - Volume 32.djvu/739

Rh at the central office is set in motion and controlled by a dial, associated with the substation set, and rotated by the subscriber. The “step-by-step” type of equipment makes use of a series of selectors in each of which contact is made by means of a central arm that can be raised to any desired level and rotated, at that level, to the proper one of a series of terminals arranged in the arc of a circle. This type of equipment is mostly used in the smaller cities and for automatic private branch exchanges. The “panel” type of equipment has been developed to a point where it is now being installed on an extensive scale in a number of the larger cities of the United States. On account of its importance a brief description is given.

Panel Type System.—The panel type equipment is so named because the multiple of the selectors is built in panels. The selectors have, in general, capacity for 500 lines or trunks. The multiple of these selectors consists of punched brass strips about 3 ft. long and one in. wide piled one above the other with insulation between. Since 3 connexions are necessary for each line or trunk, 1,500 of these strips are provided. The strips are divided horizontally into 5 groups or panels of 100 lines or trunks each and are mounted on frames having capacity for 60 selectors each, 30 on each side. The selector consists of a tube running vertically, close to the banks, the tube being equipped with a set of brushes for each bank. The brushes normally are held mechanically so that they do not engage the terminals. At the bottom of the tube, a friction clutch is provided which, by engaging constantly rotating shafts, can cause the tube to be raised or lowered. The brushes are multipled together by wires within the tube, these wires being attached at the top of the tube to feeder brushes which move over insulated feeder strips. The process of selection consists in first mechanically tripping the desired brush into engagement with its multiple bank, next in moving the tube carrying the brushes upward to choose the desired group within the bank, and finally continuing the movement upward to choose the desired subscriber's line or an idle trunk within the selected group. The subscribers' lines appear on the multiple of panel type selectors known as “line finders.” The function of the line finder is to make connexion with calling subscribers' lines. It corresponds to the “A” operator's answering cord and the subscriber's answering jack in the manual system. The brushes of the line finder are attached to the brushes of a panel type “district selector” and also to the brushes of a small selector known as a “sender selector.” As soon as a calling subscriber's line has been picked up by a line finder, the sender selector

selects an idle “sender” out of a common group. When the calling subscriber dials, the pulses are registered in the sender which controls the setting-up of the connexion and is then freed. The sender may be likened to the operator of the manual system. The sender causes the district selector to choose a trunk to the desired office, or, if more than 500 outgoing trunks from the office are required, causes the district selector to pick out an idle “office selector” of the panel type which selects the desired trunk. The trunk incoming to the full mechanical office ends in the sender of an “incoming selector” of the panel type whose function it is, under control of the sender, to pick out an idle panel “connector” having access to the group of 500 lines in which the called subscriber's line may be found. Controlled by the sender, the connector then selects the called line.

Calls from a machine-switching to a manual office are completed over “call indicator trunks.” As the calling subscriber dials his call, the district or office selector picks out an idle trunk to the desired office. This trunk ends in a plug before a “B,” or incoming trunk operator in the called office. When a call appears on that trunk, the “B” operator depresses a display key associated with that trunk, whereupon the number which is desired in that office is quickly transferred by the sender to a bank of numbered lamps appearing before the “B” operator, and the “B” operator thereupon plugs the trunk into the desired subscriber's line.

For completing calls from a manual to a full mechanical office “key indicator mechanism” is employed. This is a mechanism which indicates to the “A” operator an idle trunk to the desired office which ends at that office at an incoming selector. The “A” operator by using a small 10-button key-set is enabled to control the incoming selector to make connexion through the aid of connectors with the called line desired.

Note: In the case of countries the boundaries of which have undergone change, the figures for each year represent the number of telephones within the boundaries of that year.

Other Improvements.—In the United States there is a large and growing use of the telephone for communications essential to the operation of both steam and electric railways. The problem of minimizing the disturbing effect upon telephone circuits produced by induction interference from electric light and power circuits has resulted in careful, coordinated work by the power and telephone engineers. Important improvements have also been made in local and toll line operating efficiency.

Organized research has not only enabled the limits of telephony to be greatly extended but, at the same time, improvements and economies have been made in every department of the business.