Showing papers in "Journal of the Institution of Electrical Engineers in 1924"

A new network theorem

Abstract: A generalized theorem is given of which the well-known three-ray star-mesh transformation is a particular case The formula is of a simple nature, and a great saving of labour results from its use Three examples of application of the theorem follow: (1) To find the effective conductance between two points in a complicated network; (2) to find the effect of earth admittances on the balance of a Wheatstone bridge; and (3) to simplify the capacity network in the four-wire telephone cable and obtain the effective unbalance capacity

Power circuit interference with telegraphs and telephones

Abstract: The subject is of interest to the designer, the power engineer and the communication engineer, and the time appears to be ripe for a review of the situation. The paper is a general survey of the position, describing the features in the modern generation and distribution of electrical energy which are responsible for interference with telegraph and telephone circuits, emphasizing the importance of harmonics in this connection, and indicating some known remedies which may be applied to power plant at its source when trouble is experienced.The improvements to be expected by the transposition of wires forming overhead power and telephone circuits are described and also the practical applications. The separating distance and length where power lines and communication circuits run parallel is next considered with reference to inductive effects from (a) the balanced voltages and currents, (b) the residual voltages and currents, and (c) the abnormal occurrences due to short-circuits, etc.The balance of telephone circuits is then dealt with. Telephone circuits in this country are designed to be free from disturbance from telegraph circuits and telephone circuits carried on the same routes. It is considered that a balance which will ensure this is the most that can be expected in such a system as a public telephone service, and if disturbance from power circuits is experienced with such balanced conditions on the telephone circuits, then the injurious affection is excessive. Remedies which may be applied to telephone and telegraph circuits are described.A review of the problem and general experience in this country is then given under the headings of tramways, railways (direct-current and single-phase), and electric light and power systems, with an account of certain remedies and a discussion of the lay-out of power systems as influencing interference. In the author's opinion the solution of the inductive problem is mainly one for the designer; the power engineer, the telephone engineer and the protective-gear engineer are also concerned in that order.In an appendix Prof. E. W. Marchant describes a method of removing ripples from mercury-arc rectifiers.

The rise and distribution of temperature in small electrical machines

Abstract: The paper deals with the factors influencing the shape of the heating and cooling curves of coils and electrical machines. The thermal conductivity of the insulation is calculated by alternative methods from the distribution of temperature in coils: and the effects of different temperature-rises and of different air velocities upon the thermal conductivity and upon the temperature-rise per watt are considered. A new arrangement for measuring the mean temperature-rise of field windings is described. The temperature-rise of the field windings is deduced in terms involving the speed and the armature loss for a number of different machines, and a comparison is made of the temperature-rise as measured by resistance with that measured by thermometer.

The shape of pole-shoe required to produce a sinusoidal distribution of air-gap flux density

Abstract: Considerable interest has recently been shown in the complete elimination of all harmonics from the induced E.M.F. wave-form of an alternator. The methods usually employed in practice for the purification of the wave-shape fall into two classes: (a) in which certain harmonics are removed by proper choice of the characteristics of the armature winding, and (b) in which an endeavour is made to attain the desired end by producing a sinusoidally distributed flux round the air-gap. The object of the present paper is to give the theory of a commonly used method of the second class, in which the reluctance of the gap from point to point is adjusted to the value proper to the sinusoidal flux by shaping the pole-face to give a gap of varying length, longer at the tip than at the centre of the pole. The problem is solved for the case of a smooth-core armature, or for one in which the slots have been closed by means of magnetic wedges, such as would be used in a wave-form standard. Flat and circular armatures with any number of poles are dealt with, the shape of the pole-shoes and the permeance of the gap being determined in each case. The paper concludes with a numerical example illustrating the theory given in the text.

The whirling of shafts

Abstract: The paper is an attempt to explain the phenomenon of whirling by proving that it is essentially a case of vibration, and obeys the laws of vibration, especially those relating to the phase change between the disturbing force and the resulting displacement. Whilst acknowledging that the critical speed of whirling is usually calculated by the same expressions as those which give the natural time of vibration, it is maintained that the identity of the two phenomena is not sufficiently recognized. To prove this, the case of the spring-controlled governor is cited, which, according to the ordinary treatment of whirling-equating the centrifugal and the elastic restoring forces-should come in again above a certain speed. A description is given of experimental verifications of the theories put forward.

Some researches on the safe use of electricity in coal mines

Abstract: The limiting electrical conditions under which ignition of coal dust and firedamp may occur have been worked at in the author's laboratory for the past 14 years, as part of a general investigation of the mechanism of ignition of gases. The paper contains an account of some results of practical interest, and deals with the electrical ignition of coal dust alone, coal dust with gas present, of methane and associated gases by disruptive sparks, steady or impulsive, transient arcs, direct or alternating, with varied voltage and frequency, in all percentages of mixture with air. Examples are given of the application of the results to promote safety in mines, notably the influence of currents of higher frequency than usual, of safety bells, safety lamps which give an alarm signal when dangerous mixtures of gas occur, and an improved earthing plate.

The design of apparatus for the protection of alternating-current circuits

Abstract: The paper deals with differential protective systems, particularly those suitable for use where there is a neutral earthing resistance.The operating conditions, and their influence on methods of design and construction, are discussed, and improved means of practically applying the various protective circuits are described.The superiority of biased systems, in principle, is accepted as having been established, but the objections to a more complicated relay are recognized. The author advocates the use of simple relays only, in order to restrict to a minimum the employment of moving parts associated with small amounts of power.The necessary functions peculiar to biased and certain other protective systems have led to the use of balanced beam, dynamometer, and other special forms of relay The paper describes how these features may be achieved with a simple relay and a static device called a “biasing transformer.”Current transformers and simple types of relay are briefly dealt with, and a further static device called a “phantom auxiliary switch” is described.The application of the foregoing apparatus to actual protective circuits is described in the second portion of the paper, and attention is drawn to the advantages of dealing with earth faults and line faults independently.

Some radio direction-finding observations on ship and shore transmitting stations

Abstract: An account is given of some experiments conducted with two direction-finding receiving stations taking observations on ship transmitters located at various positions in the North Sea between England and the Continent. Damped waves of length 450 m were employed, as given by the ordinary spark transmitting sets with which the ships were fitted. The results obtained are analysed in detail and the various errors encountered are described. The effect of the various local errors to which a direction-finder is subject is not, however, studied in any detail, the object being rather to examine conditions prevailing across the sea, as compared with those across land. By careful attention to certain details various sources of error present in the earlier experiments were eliminated, with a resultant improvement in the overall accuracy. The general conclusion drawn from the experiments is that when the path of transmission is entirely over sea and in a direction making an appreciable angle with the coast line so as to be free from coastal refraction effects, the accuracy of radio direction-finding is sufficient for many navigation purposes up to ranges approaching 100 miles. Observations taken on various land stations with spark transmitters on wave-lengths of 450 and 600 m show that for greater distances over sea the variable ?night? errors encountered are of a much higher order. When the propagation of the waves is entirely over land the corresponding variations are encountered at shorter distances.

Some experiments on the screening of radio receiving apparatus

Abstract: The chief object of the investigation was to obtain quantitative information on the effect of screens, of which the dimensions are small compared with the wave-length of the waves they are intended to screen. Some preliminary experiments are described, in the first of which the screening effect of a totally closed iron tank was investigated by an operator inside it with a frame coil receiver; and in the second an investigation was made on the effect of screening a square frame receiving coil by totally enclosing each of its four sides in a metallic envelope. A description is then given of the method employed in subsequent experiments of measuring the effect of screens based upon the employment of two receiving coils connected in opposition. This is followed by an account of measurements made by this method on some specially constructed screens of straight wires, open and closed loops, and wire-netting, etc. These measurements demonstrate the important part played by closed circuits in screening the magnetic field of radio waves, and show that an effective screen may be constructed of wire-netting. In this connection a description is given of the screening of the interior of a hut with this material to protect direction-finding apparatus within it from ?direct pick-up.? Further experiments, in which the effect of screens on the electric field of the waves is measured, are then described. Three types of screen investigated were shown to reduce substantially the electric field without affecting the magnetic field, while, later in the paper, it is shown that a fourth type will screen the magnetic field but not the electric field. A cage of the former type is then described which is now in practical use as a means of eliminating ?antenna? effect from a single-coil direction-finder contained within it. A discussion of the experiments follows, some explanation of the various effects being given, and conclusions are drawn as to the general principles of screening and as to the essential points to be attended to in the design of efficient screens for various purposes.

Atmospherics and their effect on wireless receivers

Abstract: The paper contains a mathematical analysis of the effect of an atmospheric upon a wireless receiver. The choice of functions representing the atmospheric has been governed by information contained in Proceedings of the Royal Society, A, vol. 103. The effect of each of several wave-forms is calculated and the results are compared. The effect is calculated of an atmospheric acting respectively on a tuned antenna, on an aperiodic antenna, on a tuned loop aerial, and also the combination of each of these with a selective amplifier. In each case a formula is derived which gives the ratio of signal to atmospheric, and it is shown that for a given wave-length these formulae are identical. The possible advantage to be got from the use of an aperiodic antenna or of a slightly distuned periodic circuit is considered in detail. Finally, the rectified current produced by the atmospheric is compared with that produced by the signal, and it is shown that the decrement of the aerial should always be reduced to the point where ringing or bad shaping of signals commences.

Theoretical and experimental study of the electric field in an electrolytic cell

Abstract: The paper has for its principal object the theoretical and experimental investigation of the electric field produced when a current flows in an electrolytic cell having plane parallel electrodes. On the theoretical side a general expression for the electrolytic conductance is derived in terms of the stream flux and the potential difference between the electrodes, and is applied to special problems of complex transformations. The distribution of the electric stress along the surface of parallel plane electrodes is also determined mathematically (see Section 1). The experimental work has been planned with two objects in view : (1) To verify the theoretical results by means of experiments in which the mathematical conditions are represented with sufficient and reasonable accuracy (see Sections 2 and 3) ; and (2) to apply the theoretical results for the solution of certain practical questions, and to investigate experimentally how far such applications agree with test results (see Section 4). Both objects have been in some measure satisfactorily attained. On the side of electro-physics the validity of Ohm's law is proved for any elemental current in the electrolyte by a direct experiment devised for this purpose (see Section. 3). The significance of this result lies first in its providing tangible evidence of the coincidence of stream lines with lines of force everywhere in the electrolytic medium, and secondly, in its proving that under certain conditions an electrolytic may be taken to represent a perfect homogeneous dielectric with something approaching mathematical precision. As an example of the practical application of this theorem, a method is derived for calculating the conductance of such apparatus as metal-refining and simple electro-deposition tanks, and certain types of liquid rheostats. A practical chart which is provided for this purpose may prove useful to the designers of such apparatus (see Section 4). Furthermore, the results obtained may find a field of application in the measurement of electrical and magnetic phenomena where the distribution of energy is analogous to that shown to exist in the electrolytic cell, as, for example, the capacity of an irregular system of conductors or the eluctance of a magnetic circuit.

Development of the Bellini-Tosi system of direction finding in the British mercantile marine

Abstract: The paper gives an account of the various sources of error which have been encountered during the development of the Bellini-Tosi system of wireless direction-finding as an aid to navigation in the British mercantile marine. These errors can be isolated and their causes removed one by one.The application of the instrument to the purposes of navigation is also dealt with, and the possible errors, external to the ship, which may be encountered- are considered.A table is attached showing the high degree of accuracy which has now been attained.

The effect of the shape of the transmitting aerial upon observed bearing on a radio direction-finder

Abstract: For the explanation of the variable errors encountered chiefly at night on radio direction-finders, modern theory implies the reception of waves arriving at an appreciable angle of elevation and polarized so that the magnetic field has a horizontal component in the plane of propagation. Previous experimenters have shown that such abnormally polarized waves can be radiated from certain kinds of aerials when situated at a considerable distance from the earth's surface, as in the case of an aeroplane transmitter, and that such waves when arriving at the earth's surface can produce very appreciable errors on a radio direction-finder. The experiments described in the present paper were carried out to determine to what extent the emission of these abnormally polarized waves by aerials of different shapes from a ground transmitting station was responsible for the frequent occurrence of variable "night" errors at a ground direction-finding station. It is concluded that the frequency and magnitude of these errors are independent of the shape of the transmitting aerial, within the limits of this investigation, and that the use of a source of radiation which is, as far as possible, only polarized in the normal manner, in no way diminishes the night effects generally experienced.

Inductive interference with communication circuits

Abstract: The paper discusses interference between power circuits and telegraph and telephone circuits. A distinction is made between “radiation” and “induction.” The former cause produces both electric and magnetic effects and is used in radio-telegraphy. The conductivity of the earth, which is not a homogeneous body, should be taken into account. Experiment, however, shows that in radio work we can get approximate solutions by assuming that the earth is a non-conductor and that its inductivity is unity. Making this assumption, it is shown that the intensity of the “radiation” field in simple cases falls off inversely as the distance and inversely as the wave-length. On this assumption also the intensity of the induction field, whether electrostatic or electromagnetic, obeys this law in a few cases. It is pointed out that a balanced three-phase system produces both electric and magnetic rotary fields in its neighbourhood, the amplitudes of which fall off according to the inverse square of the distance. A three-phase four-wire system, however, may produce serious interference with telephone systems even when the phases are balanced. In an appendix an easy method of finding the numerical value of the mutual capacity coefficient between two spheres is given.

Periodic trigger reception

Abstract: The phenomenon of oscillation hysteresis exhibited by simple triode generators is discussed theoretically. A practical method of reception in which this phenomenon is utilized in a periodic manner is described. This method, which for convenience may be termed “periodic trigger reception,” is suitable for continuous-wave and spark signals.A low-frequency electromotive force is introduced in the grid circuit of a simple triode oscillator in such a way that high-frequency oscillations are not produced in the absence of an incoming signal. A small signal of high frequency is, however, sufficient to produce a train of free oscillations once every low-frequency cycle. Such free oscillations are rectified by the triode, and a signal of the impressed low frequency is heard in the receiver telephones.The system differs fundamentally from the Armstrong super-regenerative receiver in the following respects:—(a) The ordinary oscillation-hysteresis characteristic is followed in a “quasi-stationary” manner;(b) No self-oscillations are produced in the system in the absence of an incoming signal; and(c) The amplitude of the telephone signal is practically independent of the amplitude of the incoming signal and not proportional to it as in super-regenerative receivers.It has been discovered experimentally that oscillations may be maintained in a simple triode generator with extreme negative grid potentials such as would cut off the anode current entirely under normal conditions. A simple theory for the conditions necessary for such maintenance is given. An oscillator of this type is very efficient in that anode and grid currents are only permitted to flow for a very small part of the cycle.

The future of main-line electrification on British railways

Abstract: Extensive main-line electrification has hitherto been considered to be a rather remote possibility in Great Britain, it being supposed that the capital expenditure involved and the price of current would be prohibitive. The great superiority and simplicity of the electric as compared with the steam locomotive as a machine has not been realized. The author shows that the probable density of traffic in ton-miles on the main railway arteries is so great that the cost of energy, including all capital charges, will not be prohibitive, also that the cost of the electric locomotives will not be a capital charge, and, further, that when the reductions in the cost of locomotive operation and maintenance (due to the simple construction of the electric locomotive) are taken into account, the total cost of locomotive operation is substantially less for electricity than for steam. The conclusion is. that though it has only been possible to base the argument on general statistics, the problem merits closer and more immediate investigation than has hitherto been accorded to it.

The atkinson-type repulsion machine as a motor and generator

Abstract: The following notes are the result of experiments made by research students, under the direction of the author, on a 25-period single-phase Atkinson-type repulsion motor in the Laboratories of Applied Electricity, Liverpool University. The work involved in preparing Part I was carried out by O. J. Crompton, C. G. Ramsay and E. W. Young. That entailed in the preparation of Part II was performed by T. W. Dann.In the first place a vector diagram for the motor, in line with the standard diagram for a single-phase series commutator motor, has been developed. It is then shown how, by means of bruoh shifting, the motor may be made to operate as a generator.It is obvious that regenration by means of brush shifting has limitations in its practical application. To obviate these limitations regeneration with a fixed brush-position and a reversed field coil was investigated. This was found to be a function of the speed and could not be varied independently. Finally, by injecting an E.M.F. into the armature short-circuit, with fixed brush-position and reversed field, it has been found possible to control the amount of power regenerated at any given speed.

Iron losses in d.c. machines

Abstract: In Section I the iron loss in six d.c. machines with undistorted flux has been analysed to determine:? (a) The effect of high flux-density upon the hysteresis loss. (b) The hysteresis and eddy-current exponents over a wide range of flux density. (c) The ranges of frequency and of flux density over which the total iron loss can be represented by an expression of the from k1fxBy. (d) The values of k1, x and y 0.018 in lohys laminations. (e) The agreement between values of iron loss calculated from k1f1.5B2 with the measured values over wide ranges of frequency and of flux density. In Section 2 the increase of iron loss with load is examined. The iron loss due to the commutating-pole flux is considered, and a method of calculating the iron loss for any degree of distortion of the main flux is suggested. The values calculated by this method are compared with those measured on a dynamo in which different degrees of distortion were obtained by means of a compensating winding.

The current-carrying capacity of solid bare copper and aluminium conductors

Abstract: The current-carrying capacity of bare copper and aluminum conductors is limited by the heating effects produced by the current. The dissipation of this heat is a function of the temperature elevation above the surrounding air, the size of the conductor, and the intrinsic condition of the surface. A cooling-curve method is described which was used in determining the coefficient of heat emissivity for a series of small sample lengths of round and flat copper and aluminium conductors of various sizes and surface conditions. This was checked by the direct electrical heating of longer lengths, which showed the two methods to be in close agreement. Tables for the current-carrying capacity of copper and aluminium busbars have been deduced from the constants obtained and are given in the paper.

Transmission maintenance of telephone systems

Abstract: This paper is divided into two parts. The first, part deals with the broad aspects of modern telephone transmission maintenance, the economic justification for good maintenance, and the general lines upon which it should be carried out. The second part gives the theoretical considerations leading up to the development of a series of testing instruments, by means of which transmission losses in any part of the telephone system can easily be measured. This part also contains brief descriptions of the instruments and their use.