William Noble

“ Codes and Sending Machines.—Codes, abbreviations, typewriters and automatic sending machines are now widely used by operators to increase the capacity of manually-operated telegraph circuits. The automatic machine is merely a vibrating reed mounted in a convenient and portable manner, adjusted to vibrate at telegraph speeds and provided with contacts for controlling the telegraph circuit. A movement of the controlling lever in one direction causes the instrument to transmit a succession of dots, the number depending on the length of time the lever is thus held. ”

“ In fact it may be said that telephonic speech is now possible over any length of wire circuit. Speech through submarine cables is, however, still limited to comparatively short distances.
The most important application of the telephone repeater, and one in which the greatest economies are possible, is in the internal communications of a country. For instance, in order to provide telephonic communication between, say London and Manchester, Leeds, Newcastle and Glasgow, it has hitherto been necessary to erect line conductors weighing on the average 600 lb. per circuit mile. ”

“ Loud-Speaking Telephones.—By the use of vacuum-tube amplifiers in connexion with specially developed transmitters and receivers, supplemented by large projecting horns, the human voice may be magnified thousands of millions of times so that a public speaker can make himself heard by a vastly greater number of people than ever before. By the use of apparatus and methods of this kind developed by the Bell Telephone System, President Harding's inaugural address in 1921 was heard by over 100,000 listeners standing in an open space of more than 10 ac. ”

“ 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. ”

“ The apparatus comprises two units, a keyboard transmitter, and a printer, which are mounted on one base to form a very compact combined sending and receiving instrument. The keyboard is arranged as for a standard typewriter, and is a direct-sending instrument the keys of which when operated allow a camshaft to revolve opening and closing the line circuit according to the 5-unit code. Starting and stopping impulses are sent over the line to start and stop the selecting mechanism of the printer so that from transmission point of view the system has actually a 7-unit code. ”

“ Automatic Systems.—The idea of automatic telephony is to substitute for the operator of the manual exchange an electromechanical or other switching system, which, controlled in its movement by the action of the subscriber, will automatically select, connect and disconnect circuits as desired. The process of machine switching consists of successive group selection. ”

“ As in the case of the Creed and other automatic systems there are several perforating operators and one transmitting operator at the sending station. The prepared tape from the keyboard perforators is passed through the transmitter over five selecting needles, controlling their upward movement and determining the polarity of the current impulses sent to line during each revolution of a brush over the five segments of a distributor. The sending distributor brush arm is driven by a shunt wound motor whose speed is kept steady by means of a heavy flywheel mounted on the spindle. ”

“ There are two principal types of machine-switching equipment, the “step-by-step” and the “panel” type. In both the apparatus 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. ”

“ Considering only the former case, the machines are operated by impulses originating in a subscriber's telephone. The standard impulse now in general use is a disconnexion of the subscriber's loop (the loop having been closed in the first instance by the lifting of the subscriber's receiver) . The train of impulses corresponds to the digit signalled. For instance, the signalling of the digit 7 would constitute a series of seven disconnexions of the subscriber's telephone loop as indicated in fig. 1. It will be seen that a complete impulse is 57% of the total cycle. ”

“ Both the Relay Automatic Telephone Co. and the North Electric Co. manufacture automatic systems which do not use mechanism as usually understood. The systems consist of aggregations of relays, combined so as to provide a number of connecting or “trunking” paths through the equipment. In the case of the Relay Automatic Telephone Co.'s system the calling subscriber operates his dial in the usual way and at the same time finds an “outgoing” trunk. ”

“ Loading Coils in Exchange Service.—Many thousands of trunk circuits in multi-office exchanges and circuits connecting large cities with suburban points have been equipped with loading coils, providing transmission of such a grade as would require from 5 to 10 times as much copper in the cable circuits if loading were not employed. Loading coils have been materially improved by constructing the cores of several rings, each of which is made by compressing finely divided particles of iron with a binding material which acts as insulation between the iron particles. ”

“ Simultaneous transmission of several telephone currents is accomplished by means of selective apparatus by which one particular receiving channel is made easily receptive to one particular set of high frequency currents and, at the same time, acts substantially as a barrier to the currents of other high frequencies which are carrying telephone conversations other than those which the channel in question is designed to receive. ”

“ A type of cable known as the “quadruple pair” was introduced. In this type the conductors are lapped with insulating paper twinned together in pairs, and are arranged in “cores” each containing four twisted pairs laid up together around a centre, usually of yarn, forming a “quadruple pair” core. The cores are laid up together to the number required and sheathed with lead. This type of cable was a great improvement on the earlier “twin” cables, and permitted the formation of a superposed “phantom” circuit on two physical circuits. Diagonal pairs in the same core are selected for superposing. ”

“ 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. ”

“ Speeds as high as 50 to 60 six-character words per minute per channel are maintained by operators. The multiplex printer system has thus greatly increased operators' loads as well as the number of telegraph channels which can be obtained from each line circuit. Means have been developed for economically extending the single channels of a multiplex-printer system from the multiplex terminal station to branch offices. ”

“ The running down of the calling device in conjunction with the operation of the exchange switches controls the action of the latter by means of an electrical circuit established through the setting of the levers so that the exchange switches are made to trunk, hunt and find lines in a manner corresponding to the lever setting. The mechanism at the exchange is driven by a motor and can therefore be provided with robust contacts. ”

“ The hard-drawn copper wire of high conductivity (invented by T. B. Doolittle in 1877) is invariably used; and wires weighing from 150 to 800 lb. per mile have been employed. It was necessary to carry the long-distance lines through underground cables in the approaches to large English cities, but owing to the inefficiency of cable wires as compared with aerial wires for speech transmission, the length of underground cable sections was kept down to a minimum. ”

“ The weight per nautical mile of dielectric is the same in both cables, but each copper conductor of the former weighs 300 lb. per mile as compared with 160 lb. per mile in the latter. The transmission efficiency of the cables is practically equal, but the continuously loaded cable provides an additional circuit by superposing. Experiments conducted on this cable in 1914 proved the possibility of obtaining four circuits from a continuously loaded 4-wire submarine cable by introducing an improved method of balancing the electrostatic capacity of the conductors. ”

“ A modern telephone repeater for insertion at an intermediate point in a long telephone line consists essentially of two thermionic tube amplifiers, one for the up and one for the down side of the line circuit, associated with apparatus for balancing the line circuits for duplex working, the telephone circuit being necessarily a duplex circuit. ”

“ The circuit is provided with through-line ringing of a type which is free from interference and there is a superimposed telegraph circuit capable of forming a link in a duplex wire telegraph circuit. The volume and quality of telephone transmission are so good that the radio link is regularly connected, whenever required, with long-distance wire circuits. ”

“ It was not until the development of the 3-electrode thermionic tube had reached the stage of commercial production for wireless telegraphy purposes in 1913 that the problem of the telephone repeater could be solved. Since that time progress has been so rapid as to cause almost a complete revolution in long-distance telephony. ”

“ Machine-Switching System.—A retrospective examination of the manually operated switchboard discloses the fact that the tendency of development has been continuously in the direction of increasing the number and extent of the operating functions which are performed electro-mechanically and likewise decreasing the amount of time required of the operator for the handling of the connexion. ”

“ The Kleinschmidt Electric Co. of New York have recently designed a very compact column printer which may be used on Western Electric multiplex circuits. As in the Murray printer, there are five selecting combs which are operated by electromagnets. When the combs have been positioned, during the reception of a character, certain slots in the combs are thereby brought into alignment allowing a pull-bar attached to one extremity of the required type-bar lever to fall into them. ”

“ 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. ”

“ One of these systems employing motor-driven distributors at each station is shown in fig. 6. The sending distributor at one station and the receiving distributor at the other are indicated in detail. The distributor brush arms driven through a friction clutch are normally held stationary by a latch and make one rotation for each character transmitted or received. A start impulse, usually the opening of the line circuit for a brief interval, releases the receiving start latches so that all receiving distributors start rotating. ”

“ The Booth-Willmott-Baudot keyboard perforator punches the 5-unit code transversely on a paper tape which is of the same size as that used in the Murray and Western Electric instrument (see hereafter) , enabling the transmitters of either of these installations to be used in conjunction with it. Switches are provided on the instrument tables so that any particular arm of the multiplex may be worked automatically from a transmitter fed by a Booth-Willmott perforator or by direct-sending from an ordinary Baudot keyboard sender at will. ”

“ In the case of the receivers, a small series motor with belt drive is used, while the distributors are driven by the La Cour phonic motor. In both cases a considerable saving is obtained in first cost and maintenance expenses with the advantages of more satisfactory working and greater facility in changing apparatus when necessary.
The Carpentier method of automatic transmission on Baudot circuits has recently been reintroduced in the British Post Office, but with several important improvements. ”

“ As these perforators prepare Wheatstone slip their mechanism is necessarily much more complicated than that of keyboard perforators designed for a 5-unit code, in which all letters are of the same length, because in the former a differential feed varying from two-tenths of an inch to over one inch is required owing to the varying length of the letters. These machines will work as fast as a typewriter, but 80 words per minute is regarded as the limit for practical purposes. ”