Telephone Plant in the United States

Loading, in telephony, is accomplished by inserting coils of wire, wound on toroidal iron cores, into the telephone line at regu lar intervals. In addition to reducing the attenuation of the electri cal waves, loading has another beneficial effect. Telephony requires the transmission of waves having frequencies ranging from 200 or 30o cycles per second to more than 2,000 cycles per second. Plate I., fig. 7, is a photograph of an oscillogram showing the electrical waves produced when the word "telephone" was spoken into a transmitter. Good voice reproduction from the receiver requires that the complex wave shape of speech, including the entire "band" of frequencies, must be transmitted over the line with as little distortion as possible. Loading assists to an important de gree in accomplishing this. The principal fields of application for loading are (I) in the case of the longer trunk circuits in terconnecting the central offices in the larger cities and (2) in association with telephone repeaters permitting the use of cables, instead of wires spread out on crossarms, for the groups of cir cuits that link distant cities together.

The latest improvement in loading coils is the result of a basic scientific discovery by G. W. Elmen, in the Bell Telephone Laboratories, of a material called "permalloy" having new and unusual magnetic properties peculiarly suited to the requirements of telephony. The permalloy used in the cores of loading coils is an alloy of about four-fifths nickel and one-fifth iron, specially heat treated. The use of this material has made possible a reduction in the size of loading coils to about one-third of their previous size, and a further reduction in size based on an improved alloy, is in process. In spite of the more expensive material, the cost of coils is reduced, and also. due to the reduc tion in size, smaller cases can be used to contain the coils.

Repeaters.

As the telephonic currents pass over the line, they decrease in strength so that, after travelling a distance which varies with the character of the line, they become too weak to actuate properly the telephone receiver. The idea of inserting one or more repeaters in a line for the purpose of reinforcing the tele phonic currents from some local source of energy is almost as old as the telephone itself, but many years elapsed before the quest for a satisfactory repeater culminated in success. The solution of this problem involved the production of a repeater element which, when actuated by weak telephonic currents, is capable of emitting greatly strengthened currents without appreciable loss in the clearness of voice transmission. A so-called "mechanical" element was first used but this presented inherent limitations owing to the mechanical inertia of the moving parts. A satis

factory solution of the problem was found by improving and utilizing the three-electrode vacuum tube in which the weakened speech currents control a local source of energy by means of a practically weightless stream of electrons. The three-electrode vacuum tube of De Forest, prior to its development for use in telephone repeaters, was employed for radio telegraph reception. In the form in which it was used for that purpose it was found to be quite unsuited to meet the rigid requirements of telephony. A large amount of research and development work was required before a satisfactory tube could be produced. The continuing work on the development of tubes has involved the fundamental study of electronic emission from heated filaments together with the study of proper materials and dimensions for the elements of the tube, means for securing a high vacuum and other necessary conditions.

To illustrate the progress that has been made it can be stated that the life of the vacuum tube most commonly used in 1917 was about i,000 hours. Tubes of a new type first introduced in 1927, have a life of fully 25,00o hours, which is equivalent to about three years of continuous operation. In addition, the first cost was reduced by more than one-half and the power required to heat the tube filaments was also reduced to less than one-half of that required in 1917. The amount of testing needed to secure good operation was likewise decreased.

The very large use of vacuum tube repeaters is shown by the fact that at present (1936) there are over one hundred thousand repeaters using about three hundred thousand vacuum tubes in the telephone plant in the United States. The repeater element would have been of little use without the concurrent development of efficient and convenient circuits and auxiliary equipment to enable the repeater element to be associated properly with the line circuits. This problem was partially solved as early as 1904 when an elementary form of mechanical repeater was first used com mercially. The development of a satisfactory repeater has car ried with it the necessity for extensive investigations of the characteristics of telephone lines in which the repeater must work. Not only must the electrical characteristics of the line be accurately known but the use of repeaters has a bearing on the type of line circuit selected and on the type of loading em ployed. A long cable with its repeaters, loading, associated switchboard apparatus, echo suppressors and temperature con trolled transmission regulators must be engineered as a unit in order to obtain the best overall results.