Showing papers in "Bell System Technical Journal in 1928"

Transmission of information

Abstract: A quantitative measure of “information” is developed which is based on physical as contrasted with psychological considerations. How the rate of transmission of this information over a system is limited by the distortion resulting from storage of energy is discussed from the transient viewpoint. The relation between the transient and steady state viewpoints is reviewed. It is shown that when the storage of energy is used to restrict the steady state transmission to a limited range of frequencies the amount of information that can be transmitted is proportional to the product of the width of the frequency-range by the time it is available. Several illustrations of the application of this principle to practical systems are included. In the case of picture transmission and television the spacial variation of intensity is analyzed by a steady state method analogous to that commonly used for variations with time.

Distortion correction in electrical circuits with constant resistance recurrent networks

Abstract: Constant resistance recurrent networks, that is, networks whose iterative impedances are a pure constant resistance at all frequencies, form here the basis of a method of distortion correction which is applicable to any electrical circuit. The paper takes up first the general problem of distortion correction, then this method of correction and its application in the following Parts and supplementary Appendices.

The diffraction of electrons by a crystal of nickel

Abstract: This article is taken from the manuscript prepared by the author for his address at the joint meeting of Section B of the American Association for the Advancement of Science and the American Physical Society on December 28, 1927, at Nashville, Tennessee. An account of this work giving fuller experimental details is given by Davisson and Germer in the December, 1927, issue of the Physical Review. These experiments are fundamental to some of the newer theories in physics. Until they were performed, it could be said that all experimental facts about the electron could be explained by regarding it as a particle of negative electricity. It now appears that in some way a “wave-length” is connected with the electron's behavior. The work thus shows an interesting contrast with the discovery of A. H. Compton that a ray of light (a light pulse) suffers a change of wave-length upon impact with an electron, the change of wave-length corresponding exactly to the momentum gained by the electron. Until Compton's work, all the known facts about light could be explained by thinking of light as a wave motion. The Compton effect seems to prove the existence of particles of light. Physics is thus faced with a double duality. Compton showed that light is in some sense both a wave motion and a stream of particles. Davisson and Germer have now shown that a beam of electrons is in some sense both a stream of particles and a wave motion. At the same time, theoretical advances have been made which seem to pave the way for an understanding of this curious situation. A general account of these new developments was given by K. K. Darrow in his series “Contemporary Advances in Physics” in the Bell System Technical Journal for October, 1927. Some remarks on the relation of the Davisson and Germer experiments to the new mechanics were given in this article, p. 692 et seq. — Editor.

A method of rating manufactured product

Abstract: This paper outlines a method of rating manufactured product. In the particular form here described, the rate has been found very useful for measuring the quality of communication equipment and materials entering the plant of the Bell System. While the primary object is control of quality of finished product, it is proving useful for measuring the workmanship of individual operators and groups of operators engaged in similar production work. Particular attention is directed to the statistical aspects of the rate to show how it can assist in controlling quality.

Harmonic production in ferromagnetic materials at low frequencies and low flux densities

Abstract: When a multi-channel communication circuit includes a non-linear element such as a ferromagnetic core coil, distortion of the wave form impressed upon the circuit is produced. In terms of the single frequency components, this distortion is manifested in the appearance of new components. This distortion may give rise to the reduction of quality in any channel, and it may also introduce crosstalk and interference, which consists of new frequencies not present in the impressed wave of any channel under consideration, produced by independent channels. In view of the recent increased use of multi-channel systems, it has become necessary to investigate the effects of this type of distortion, to determine the dependence of this distortion upon the properties of the magnetic materials constituting the cores of inductance coils and transformers, as well as upon the circuit impedances, and to determine those constants of core materials which are significant in the distorting process. The behavior of magnetic materials to complex waves of magnetizing force is ordinarily a highly involved process, so that a direct correlation between distortion and some of the easily measured constants of materials is a matter of some difficulty. It has been established experimentally, as a confirmation of theoretical speculations, that the third harmonic e.m.f. generated by a sinusoidal wave of magnetizing force may serve as an index of the distortion with a complex wave of magnetizing force. This relation is valid for low flux densities and for frequencies at which the screening effect of eddy currents is not important. The paper is therefore devoted to an investigation of the third harmonic production in its dependence upon the properties of hysteresis loops. These loop constants in turn are shown to be deducible from AC bridge measurements on a coil of known dimensions having a core of the magnetic material under investigation. The loop constants for a few materials are included in the text. An analogy exists between the treatments of hysteresis loop and of three-element vacuum tube characteristics which enables us to compare simplifying relations introduced by Rayleigh and by H. J. van der Bijl in the two cases. The theoretical deductions are found to be in general agreement with experiment, and are applied to a number of cases of practical interest. These include the effects of air gaps and dilution, and the choice of core material in third harmonic production by inductance coils and transformers. Finally, the amount of third harmonic current flowing out of long lines is deduced with both lumped and continuous loading.

Phase distortion and phase distortion correction

Abstract: The importance of the rocle played by the steady state phase characteristics of long cable circuits has recently been emphasized in telephone and telegraph transmission. In this paper an analytical exposition of the theory of phase distortion is followed by a consideration of various methods of phase distortion correction with particular reference to terminal phase compensating networks and to the application of the lattice network to the loaded line as a terminal phase corrector.

The rigorous and approximate theories of electrical transmission along wires

Abstract: THE theory of electrical transmission along straight parallel guiding conductors is of fundamental importance to the communication engineer. In its original, and largely in its present day form, it involves only relatively simple concepts which go back to the early work of Kelvin and Heaviside. In accordance with these concepts the transmission phenomena are completely determined by the self and mutual impedances of the conductors and the self and mutual capacities (together with the dielectric leakage). As a consequence, the phenomena are completely expressed in terms of the propagation constants and corresponding characteristic impedances of the possible modes of propagation deducible from these underlying concepts.

The practical application of the Fourier integral

Abstract: The growing practical importance of transients and other non-periodic phenomena makes it desirable to simplify the application of the Fourier integral in particular problems of this kind and to extend the range of problems which can be solved in closed form by this method. Unless the physicist or technician is in a position to evaluate the definite integrals which occur, by mechanical means, he is usually entirely dependent upon the results obtained by the professional mathematician. To facilitate the use of the known closed form evaluations of Fourier integrals many of them have been compiled and tabulated in Table I. They are presented, however, not as definite integrals but as paired functions, one function being the coefficient for the cisoidal oscillation (or complex exponential) and the other function the reciprocally related coefficient for the unit impulse. This arrangement simplifies the table and promises to be most convenient in practical applications, since it is the coefficients of which immediate use is made, just as in the case of the Fourier series. Applications of the tabulated coefficient pairs to 85 transient problems are given, together with all necessary details, in Table II.

Carrier systems on long distance telephone lines

Abstract: Two previous papers before the American Institute of Electrical Engineers discussed the activities of the Bell System in the development of multiplex telephone and telegraph systems using carrier current methods. The present paper describes developments which have resulted in improvements in the carrier telephone art during the past few years. A new, so-called type “C” system is described in detail, together with suitable repeaters and pilot channel apparatus for insuring the stability of operation; the line problems are considered and typical installations pictured. The growth of the application of carrier telephone systems and their increasingly important part in providing long distance telephone service on open-wire lines are shown.

A high efficiency receiver for a horn-type loud speaker of large power capacity

Abstract: This paper describes a telephone receiver of the moving coil type which is particularly adaptable to the horn type of loud speaker and which represents a notable advance over similar devices at present available. Its design is such as to permit of a continuous electrical input of 30 watts as contrasted with the largest capacity heretofore available of about 5 watts. In addition, measurements show that the receiver has a conversion efficiency from electrical to sound energy varying between 10 and 50 per cent in the frequency range of 60 to 7,500 cycles. Throughout most of this range, its efficiency is 50 per cent or better. This contrasts with an average efficiency of about 1 per cent for other loud speakers either of the horn or cone type. Combining the 50 fold increase in efficiency with a 5 or 6 fold increase in power capacity, a single loud speaker unit of the type here described is capable of 250 to 300 times the sound output of anything heretofore available. This device is in commercial use in connection with the Vitaphone and Movietone types of talking motion pictures. As commercially produced in quantities numbering several thousand, an average efficiency of the order of 30 per cent has been realized.

High-speed ocean cable telegraphy

Abstract: The invention of permalloy and its application to submarine cables have led to the installation of transoceanic cables of many times the traffic-carrying capacity of the former non-loaded cables. This paper relates briefly the history of the development of permalloy-loaded cables and discusses certain outstanding problems concerned with their design, construction and operation. In a concluding general survey the field of usefulness of loaded submarine telegraph cables is considered. To a considerable extent the paper is a critical summary of material previously published by members of the staff of the Bell Telephone Laboratories. Its scope is indicated by the sub-titles as follows: Loaded Cables Now in Service Historical Remarks Permalloy and Its Application to Cables Principles of Design of Loaded Cables Principles Involved in Operation Apparatus for Restoration of Signals Apparatus for Automatic Operation Electrical Measurements of Loaded Cables A General Survey

The measurement of capacitance in terms of resistance and frequency

Abstract: The adaptation of a bridge circuit due to M. Wien together with apparatus and procedure is described which permits measurement of capacitance in terms of resistance and frequency with an accuracy comparable to that of the primary standards. Among its advantages over the Maxwell method commonly employed are the use of a single frequency voltage and the fact that there is no general limitation placed on the type of condenser which may be measured or on the frequency at which the measurement may be made. The method is also applicable to the determination of inductance since its unit, like that of capacitance, may be derived from the units of resistance and frequency.

Grid current modulation

Abstract: The term grid current modulator is used to describe those vacuum tube circuits in which modulation is initially produced in the grid circuit of a three-electrode vacuum tube due to the non-linear grid current-grid voltage relation. Comparison with a representative plate current modulator using the same tubes and the same plate potential shows that by modulating at maximum efficiency in the grid circuit and using the plate circuit solely for amplification, the maximum power output is increased about eight times, the power efficiency is increased about five times and the ratio of sideband output to signal input is increased approximately three times. Under these conditions more carrier input power is needed for the grid than for the plate modulator. This improved performance has been made possible by a detailed study of the fundamental processes involved and by a design of the tubes and associated equipment, such as transformers and filters, to permit these fundamental processes to operate to their best advantage. Normally modulation is also produced in the plate circuit which is shown to be out of phase with that produced in the grid circuit. By inserting high impedances to the input frequencies in the plate circuit, plate circuit modulation is prevented, and the reduction of grid circuit sideband is likewise avoided. By including in the grid circuit an impedance which is high to the desired sideband frequencies, the maximum grid sideband voltage is obtained. In this way the power and modulating efficiencies of the tube circuit are made maximum. Where modulation occurs only in the plate circuit of a tube, the sideband amplitude is proportional to the product of the amplitudes of the input frequencies when these amplitudes are small. In the present type of grid current modulator the sideband amplitude is proportional to the smaller of the two input amplitudes provided the ratio between these is greater than about 3/2. This feature makes the modulator particularly valuable in communication systems. The article concludes with an application of the fundamental principles involved to an experimental carrier telephone system in which the operating features of tubes, filters, and transformers are discussed.

The measurement of acoustic impedance and the absorption coefficient of porous materials

Abstract: Various ways of determining the acoustic impedance and the absorption coefficient of porous materials from measurements on the standing waves in tubes are discussed. In all cases the material under investigation is placed at one end of the tube and the sound is introduced at the other end. Values of the coefficient of absorption of a number of commonly used damping materials as obtained by one of the methods are given. Several types of built-up structures are shown to have a greater absorption coefficient for low frequency sound waves than is conveniently obtainable by a single layer of material.

The natural period of linear conductors

Abstract: This paper describes the experimental determination of the frequency of free electrical oscillation of straight rods and circular loops. The results agree more closely with the formula of Abraham than with that of MacDonald. For three rods whose lengths were 300 cm., 250 cm. and 227.1 cm., the ratio of wave length at resonance to rod length had the values 2.11, 2.13 and 2.13, respectively. Measurements taken upon 250 cm. rods bent into circular arcs of different radii gave values of the ratio of resonant wave length to arc length which passed through a minimum value and were virtually independent of the radius of the arc over a wide range, deviating markedly only at the extreme value of minimum radius possible and infinite radius. The extreme measured range of the ratio was 2.05 to 2.166. The wave lengths were measured upon a pair of Lecher wires and a very satisfactory meter for the rapid comparison of waves of short length was found to be a quarter wave length Lecher frame. This frame showed a constant end correction so that λ = 4(d + 3.1), d being the length of the parallel rods.

Electrical measurement of communication apparatus

Abstract: This paper describes precision high-frequency measurements of a fundamental type, special emphasis being placed on the measuring circuits rather than on the types of apparatus measured. Standards of frequency, resistance, capacitance, and inductance are discussed briefly. Bridge measurements are described for the measurement of frequency, inductance, effective resistance, capacitance, dielectric loss, capacitance balance and inductance balance. Circuits for the measurement of other high-frequency characteristics such as attenuation, gain, and cross-talk are included.

Contemporary Advances in Physics, XVI

Abstract: This is an elementary introduction to the phenomena of diffraction of waves by crystals, one of the most striking and important discoveries of the last twenty years of physics. These phenomena have proved that X-rays and electrons are partly of the nature of waves, and have supplied the best available methods of measuring their wave-lengths; while on the other hand, the study of the diffraction-pattern of a crystalline substance makes it possible to determine the arrangement and the interrelations of the atoms with a precision and fullness heretofore unimagined, which has already yielded knowledge of great value in all the fields of science and promises immeasurably more.

Bridge for measuring small time intervals

Abstract: A bridge circuit fur measuring time intervals from about one ten-thousandths of a second up to several seconds is described and its operation explained. The device is fairly accurate and easy to operate and gives the results of measurements in fractions of a second directly. Its calibration can readily be determined mathematically since it is dependent only upon the values of certain capacities and resistances used in the measuring circuit.

The present status of wire transmission theory and some of its outstanding problems

Abstract: The rapid development in the technique of wire transmission and the increasing complexity of the problems involved calls for a more adequate theoretical guide and a more rigorous transmission theory This paper gives an account, practically without mathematics, of classical transmission theory and its limitations; of the several ways the problem may be attacked more fundamentally and rigorously, and the lines along which transmission theory must be extended, as the writer has come to view the problem in the light of his own experience

Airways communication service

Abstract: THE present development of air transport is bringing out its need for adequate communication in much the same manner as the earlier development of railway operations disclosed for that industry the necessity of special communication services if speed and density of traffic were to be obtained with safety. The electric telegraph by a most fortunate coincidence was available just at the time the railways required it; and as the demand for speed became pressing the telephone was perfected. Today the railways of the country, in general, use the telegraph for administrative messages, where a written record is wanted, and use the telephone for despatching, where speed and accuracy are primary requirements.

Some general results of elementary sampling theory for engineering use

Abstract: EVERY day we base conclusions on the results of the process commonly known as “sampling.” For example, if five times in a week a man has waited ten minutes or more for his trolley at a street corner, he may conclude that the transportation facilities are poor. Or again, if a housewife has bought ten loaves of bread at a certain store and has found five of them not as fresh as might be desired, she decides that in the future she will buy her bread elsewhere. Both of these conclusions are based on an intuitive application of sampling theory. Such examples could be multiplied indefinitely.

Precision tool making for the manufacture of telephone apparatus

Abstract: There is probably no field of human endeavor in which hand labor has been more completely replaced by labor-saving devices than in the field of manufacturing. The design and employment of special tools together with semi- and full automatic machinery for operating them have reached a high stage of development and are probably more responsible than any other factors for the present age being generally referred to as the industrial age. The notable economies of present day manufacture result no more from the rapid production of parts thus made possible than from the interchangeability of these parts because of the accuracy with which they have been produced.

Automatic machine gaging

Abstract: This paper discusses the advantages to be gained in certain types of large scale production by the substitution of automatic machine gaging for hand testing. For testing carbon protector blocks, a machine has been developed which accepts all blocks in case a certain dimension lies between 0.0024″ and 0.0032″ and rejects those when the dimension is 0.0023″ or less or 0.0033″ or more. This machine will effect a saving of $8000 per year over the cost of hand gaging on an output of 4,500,000 blocks. The saving effected by another recently developed machine replacing a manual test is approximately $1200 a year on a production of 2,500,000 pieces, but a far more important consideration than this money saving is the elimination of an operation so monotonous that it was difficult to keep any operator on it for more than a brief period. The author points out that in some instances automatic machine gaging of the entire product will cost less than a sampling inspection in which there must be included in the direct cost of inspection the cost of some additional supervision and control.

Transatlantic telephony — Service and operating features

Abstract: This paper describes some of the differences in operating practice on the two sides of the Atlantic and plans which were worked out for taking account of them in the handling of commercial transoceanic calls. Difference in the language is also another problem which has required solution. Data are included giving an idea as to the extent to which the transatlantic connection was used during its first year, there having been established during this time a total of something over 2,300 connections.