In what is known as the full automatic sys tem, the subscriber puts the automatic mechan ism at the central office in motion and directs it so as to obtain the desired connection by suitably operating a movable numerical dial at tached to the base of his substation set. Fig. 44 shows a typical set with the dial. The face of the dial is shown in Fig. 44-A.
Briefly the operation is as follows: Assume connection is desired with subscriber's line 753. The calling subscriber places his finger-tip in the hole of the dial, on his set, over figure 7 and turns the movable part of the dial in the direction of the hands of a clock, until his finger reaches the crescent-shaped stop. He then releases the dial which is automatically moved hack, in ihe opposite direction, by a spring. This motion, by making and breaking a current on the line seven times, sets in opera tion in the central office an electromechanical device known as a selector, whose function it is to select some idle trunk line in the 700 group. These trunk lines terminate in other selectors of a similar type. By repeating the dialling operation, this time for figure 5, con nection is established with the fifties in the 700 group. By dialling once again, this time for figure 3, the line of the calling subscriber is placed in connection with line 753. The selectors are made in a great variety of forms. In one form the terminals of the trunks and sub scribers lines are arranged in panels, one above another, with selector rods moving vertically in front of these panels. In another form the terminals are arranged in layers around an arc, contact being established by a central arm capable of being raised to the proper level and then rotated.
In both forms suitable means are provided for automatically ringing the called subscriber's bell and for indicating to the calling party when the called line is already in use.
In the semi-automatic system the subscriber removes his telephone from the hook and trans mits the desired number by telephone to the central office operator, as in the manual system. Thereafter, however, the operator makes use of automatic machinery at the central office' to complete the connection, instead of performing the switching operations manually.
In modern telephone engineering the par ticular factors, surrounding each case where new central office equipment is to be recom mended, are carefully studied in order to de cide whether a manual, semi-automatic or full automatic system is, in view of all the circum stances, the best. The names of Keith, Lori
mer, McQuarrie and Strowger are prominent among those who have contributed to automatic switchboard development.
Telephone Lines. Early Lines Similar to Telegraph Lines.— The first telephone line was a copy of the best telegraph line of the day. It was a single iron wire using the ground for a return circuit. In addition to electrical dis turbances, coming from no one knew where, picked up by these early F rounded lines, and frequently causing noises in the telephone so loud as to destroy conversation, it was dis covered that iron wire was not as good a con ductor for the telephone current as it was for the telegraph current. The talking distance, therefore, was limited by the imperfect carry ing power of the conductor. It was realized that, if the telephone system was to grciw, something better than iron wire would be re quired. Various metals and alloys were studied and it was found that copper best met the eleCtrical requirements. But copper made into wire according to the then state of the art was too weak in mechanical strength. T. B. Doo little conceived the idea that if a copper rod could be drawn cold through a series of dies, the resulting wire would acquire a degree of tensile strength much greater than that of the soft, annealed wire which was the only copper wire then known to the trade. He conducted a series of experiments which resulted in the production of commercial hard drawn copper wire having substantially all of the favorable electrical characteristics of soft copper wire, coupled with a tensile strength nearly double that of the soft wire, and fully equal to the strength of ordinary iron wire of the same size.
An experimental line, the first long distance telephone line in the world, was constructed in 1884 between New York and Boston, carry ing two hard drawn copper wires 0.104 inches in diameter. The satisfactory conversations conducted over this line demonstrated the com mercial feasibility of the new type of wire. In the following year. 1885, another line, built between New York and Philadelphia, was equipped with 24 hard drawn copper wires of various sizes. Careful experiments conducted on this line established the properties and qualities of the new product and the relative efficiencies of different sizes and variations.