Showing papers in "Journal of the Institution of Electrical Engineers in 1933"

An investigation of the magnetron short-wave oscillator

Abstract: The possible methods of utilizing magnetrons to generate short-wave oscillations are indicated and the more important results of previous workers are described. The theoretical basis of “electronic” and “dynatron” oscillations is discussed, with particular reference to those features which can be investigated experimentally. It is shown that the wavelength of the electronic oscillations is determined mainly, if not entirely, by the electron time of transit. The general expression for the time of transit is given and hence expressions for the wavelength in terms of magnetic field strength are obtained for zero and saturated space-charge conditions. The wavelength is found to be about 36 per cent greater in the latter case. It is shown that initial electron velocity causes an appreciable reduction in wavelength in normal cases. The effect of magnetic field on space charge is considered and is found to lead to a small, possibly negligible, increase in wavelength. It is shown that the space charge is uniformly distributed when the magnetic field strength exceeds the critical value at which the electron orbits just touch the anode surface. This result has been previously stated by Hull, but Hull's deduction that the electrons travel in circular orbits round the cathode is disputed.An attempt to provide a simplified theoretical explanation of the “dynatron” characteristics of a “split anode” magnetron leads to a false result from which it is concluded that no theory will provide an explanation which does not take into account the non-uniformity of the electric fields in the two halves of the valve. The general shape of the static characteristics is indicated by means of Habann's theory, which is, however, not capable of giving proof of the existence of negative resistance in the case of a symmetrical oscillatory circuit, which is the case considered here. A qualitative explanation of the occurrence of negative resistance, i.e. of the greater fraction of the anode current reaching the lower-potential anode segment, is given.The object of the experimental investigation was to discover the nature of the fundamental relations in the electronic and dynatron types of oscillation, to compare these relations with the indications of the theory, and to apply the knowledge obtained to the production of a sufficient amount of power to be technically useful at the shortest possible wavelength.For electronic oscillations it is found that the experimental results are entirely in agreement with the theory in so far as it is applicable. In particular it is confirmed that the strength of the electronic oscillations is greatest at the “critical” relation between anode voltage and magnetic field strength, and that the wavelength of the optimum oscillation is inversely proportional to the magnetic field strength. The actual value of the wavelength and the amount of the wavelength change due to space charge both agree with the theoretical values within experimental accuracy. It is concluded that the effect of magnetic field on space-charge distribution is not great enough to affect the wavelength appreciably. It is shown that apparently anomalous results can be explained by taking into account the stray capacitances, due in particular to the glasswork of the valve, across the oscillatory circuit. These lead to internal resonance effects which, although often a nuisance to the investigator, sometimes have the advantage of enabling a relatively large output to be obtained at a particular wavelength.The fact that the greatest output is, in general, obtained with the magnetic field not exactly in the direction of the electrode axis, which has been reported by Slutzkin and Steinberg and by Ranzi, was observed independently. The existence of an optimum field angle differing from zero is found to be due to the resultant spiral motion of the electrons balancing out the effct of cathode potential-drop for electrons arriving at part of the anode surface in such a way as to increase the number of oscillating electrons.By making use of an internal resonance effect and suitably adjusting the field angle, an output of the order of 1.5 watts was obtained at a wavelength of about 24 cm.It is pointed out that the wavelength of the electronic oscillations can be expressed in the same form as the Barkhausen-Kurz equation for the triode case, and that the existence of an optimum value of anode current (approximately l/10th of the space-charge saturation value) leads, as in the triode case, to the result that the minimum wavelength obtainable without overloading the valve depends on the anode diameter and decreases with it. For an anode diameter of 3 mm the shortest wavelength at which optimum oscillating conditions can be maintained is of the order of 20 cm. The corresponding figure for the triode case (grid diameter 3 mm) is about 50 cm. The shortest wavelength actually observed was 18 cm. Oscillations of shorter wavelength have not been investigated, owing to the small power obtainable.It is shown that it is possible to obtain dynatron oscillations in a split-anode magnetron by tilting the magnetic field, and that these are distinct from both the electronic and simple dynatron oscillations. Hollmann has investigated the occurrence of oscillations of this kind in the full cylindrical. anode magnetron and found a minimum wavelength of about 20 m. It is shown here that these oscillations can be produced down to about 35 cm wavelength, but at this wavelength the effect of electron inertia is important.In the investigation of the simple dynatron oscillations, static characteristics showing the negative resistance effect are obtained. From these curves the operating characteristics of the valve are calculated and checked by comparison with a set of measured values (for a relatively low frequency). Good agreement is obtained.It is found that the energy of the dynatron oscillation starts to fall off rapidly at a wavelength which is about 4 times the electronic oscillation wavelength corresponding to the anode voltage used. This leads to a formula for the wavelength limit for dynatron oscillations.The relation between oscillation amplitude and anode voltage is discussed, with particular reference to modulation.It is found that during oscillation the anode current may exceed the original total emission. This is probably due to bombardment of the filament by electrons which return to it with considerable velocity. The exact mechanism of this bombardment is not clear and the effect is being further investigated. By taking steps to reduce this effect it has been possible to obtain an output of 60 watts from a relatively small valve at about 2 m wavelength.The shortest wavelength obtained by means of dynatron oscillations was about 30 cm. At this wavelength the power obtainable was about 0.1 watt. It is concluded that for wavelengths below about 50 cm electronic oscillations give the greater output.

Electrical methods of geophysical prospecting

Abstract: Geo-electrical methods of prospecting depend on the marked differences in the resistivity which exist between adjacent geological formations, these differences being produced by variations in the porosity of the rocks and in the nature of the saline solutions which they contain. Minerals with a metallic luistre usually have a low resistivity. The variations in resistivity are revealed by the anomalous flow through the ground of an applied current, and the work consists in determining the current flow and interpreting the anomalies in terms of geological structure. The usual methods employed are the surface-potential methods (i.e. the resitivity method, equipotential-line methods, and equiquadrature methods, etc.), and the inductive methods at high or low frequencies, in which the current distribution is examined by means of the magnetic field associatedc with it. The methods may be applied to the location of good conducting ore bodies, faults, and dykes, to the measurement of depths, and also for the plotting of subsurface contours. Under favourable circumstances sulphide bodies may be detected by the natural currents which they produce(spontandeous-polarization method).

Corona and spark discharge in gases

Abstract: Part 1 of the paper deals with an experimental investigation of alternating-current corona discharge in air and other gases, under conditions in which the effects of space charges are negligible. The results are of theoretical and practical interest, an analysis of them resulting in a theory of electrical discharge which forms the basis of Part 2 of the paper.The results of corona discharge for all the gases are found to obey accurately the empirical relationGm=Gℴρ{1+B/√(γρ)}given by Peek, Whitehead, and others. The experimental results for air, andits principal constituents, oxygen, nitrogen, and carbon dioxide, are given in detail, and the constants G0 and B are quoted for a few of the gases which are considered later in the paper in relation to sphere-gap spark discharge.Assuming Russell's theory of sphere-gap spark discharge, it is shown that the condition for this discharge isGm=KGℴρ{1+K'B/√(γρ)}where G0 and B have the same values as in corona discharge, and K, K, are constants which have the same values for all gases.Part 2 of the paper deals with sparking potentials as a measure of maximum voltages, and is a preliminary account of the work. A new formula is given for uniform-field spark discharge, and this form of discharge is proposed as an alternative to sphere-gap discharge for the measurement of voltages up to 300kV (r.m.s.), the limit of the experiments. Advantages and disadvantages of the two types are also considered. The results of an experimental investigation of uniform-field spark discharge are given, for variations of spacing and gas density.

Television with cathode-ray tubes

Abstract: This paper gives a preliminary outline of work with a new, entirely electrical, television system having no moving mechanical parts. The registration of the image is accomplished by a special device, which is a true electric eye. The device has been named the ?iconoscope,? and it consists of a vacuum tube containing an electron-emitting gun and a photo-sensitive surface of a unique type. This photo-sensitive surface is scanned by an electron beam from the gun, which serves as a type of inertialess commutator. A new principle of operation permits the storing of the energy from the light of the image and enormously increases the output as compared with that of conventional types of television scanners. The sensitivity of the iconoscope, at present, is approximately equal to that of a photographic film operating at the speed of a motion picture camera. The resolution of the iconoscope is high, much higher than is necessary for television of the highest quality. The reproduction of the image is accomplished by another cathode-ray tube with a fluorescent screen, which has been named the ?kinescope.? Here the impulses from the transmitter modulate the intensity of the electron beam, and this, in turn, is transformed by the fluorescent screen into variations of light. The scanning is linear and is synchronized at the end of each line. The synchronizing impulses are transmitted through the same channel as the picture signal. The whole system is completely automatic and the receiving sets are almost as easy to operate as the ordinary radio receivers. The paper describes the theory, characteristics, and mode of operation of the system.

The resistance of earth electrodes

Abstract: This paper, which is an abridgement of a report† prepared by the ERA, deals briefly with the most important aspects of the resistance of electrodes used for earthing electrical installations and apparatus After comparing the practice in various countries, details are given of the effect of the resistivity of the soil and the size and lay-out of electrodes on their resistance It is shown that in order to have a low resistance the electrodes should cover a large area by consisting of either strips or, alternatively, a number of small electrodes connected in parallel To facilitate the latter arrangement it is recommended that driven rod or pipe electrodes be used A low resistance may also be obtained by salting the soil around electrodes, and this is considered together with the effect of using coke breeze

The influence of mercury-arc rectifiers upon the power factor of the supply system

Abstract: The rapidly increasing application of mercury-arc rectifiers and the certainty of their exclusive use in the near future in the bulk conversion of alternating to direct current for railway electrification projects, require that more careful consideration be given to the peculiar nature of the load which a rectifier imposes on the supply system. Few electricity supply authorities appreciate that, in the case of rectifier substations, power-factor meters give no real indication of the additional extent to which the generating plant is loaded.The paper investigates the manner in which the mercury-arc rectifier draws power from the supply, with the object of ascertaining the degree to which the generation and distribution system is utilized.The general case of power production from currents and voltages of non-sinusoidal wave-form is considered, and, by introducing a new factor, viz. harmonic power, fresh significance is given to the conception of wattless power.The presence of harmonics in the current delivered by and supplied to the rectifier is briefly explained, and their modification due to smoothing equipment and to parallel connection of rectifiers is indicated.In dealing with the power-factor problem as applied to the mercury-arc rectifier, its treatment by previous investigators is reviewed. The relation between rectifier and system power factors is then discussed and an analytical method of determining the additional loading due to harmonics is developed. Convenient formulae for evaluating the resultant power factors and for expressing the harmonic loading in terms of the reactive power are finally deduced.The paper concludes with a suggested application of the foregoing method of analysis to the amendment of electricity supply tariffs.A bibliography of the available literature covering the problems dealt with in the paper is given at the end of the paper.

A new theorem for active networks

Abstract: A theorem is proved which states that any network with three accessible terminals can be replaced by a star or delta of certain e.m.f.'s and impedances. This is an extension of Kennelly's star-delta theorem, and Thevenin's theorem. The theorem is applied to find the equivalent network of a transformer and a 3-phase alternator (with coupling between windings and unequal e.m.f.'s) The sensitivity of an impedance bridge, in which a 3-winding transformer is used, is also found by the help of the theorem. Finally, the theorem is used to find the errors in voltage measurement due to earth capacitances. An important result which emerges is that, in measurements of attenuation in balanced systems, an unbalance of capacitance that occurs after the attenuation pad causes an error in the measured voltage which is not attenuated by the pad. This is in contradiction to accepted theory, but agrees with practice.

The recovery of deep-sea cable

Abstract: The kinking of deep-sea cable during repairs, and the failure of the cable to coil properly when an endeavour is made to recover it, are troubles which are as old as the art. The paper attributes these troubles to two causes?the tendency of laid-up cable to unlay under tension, and the twisting action of the bowsheave when the cable trends outboardto port or star board. An example is given of a method of calculating the tendency to unlay under tension. It is shown what are the effects resulting from a trend of the cable to port, tostarboard, and when central; also that the variation of the effects when the trend is altered follows an exponential law. It is further shown that a certain trend of the cable outboard enables torsion in the suspended cable to be avoided during pauses in picking up, thus avoiding kinking at the bottom, and that the constancy of the trend conduces to satisfactory coiling. Novel forms of sheaves are described which would facilitate keeping the cable outboard at a constant trend relative to the sheave.

A method for the investigation of the impedance of the human body to an alternating current

Abstract: A technique is described for carrying out measurements of the impedance angle of the human body. It is shown that this is a measurement of true physiological significance independent of electrode and skin effects.A study is made of the impedance angle in the normal subject and reference is made to an investigation in which it has been shown that in cases of thyrotoxicosis there is a correlation between the impedance angle and the basal metabolic rate. The variation of impedance angle in thyrotoxicosis is a reliable aid to the diagnosis of this disease.

A valve wattmeter

Abstract: The paper describes a wattmeter depending for its action on the use of two thermionic valves and a direct-current measuring instrument. Three forms of instrument are described, the first suitable for measuring the powers used in ordinary power work, and the second and third for measuring the small powers using in communication work.

The effect of the curvature of the characteristic on the frequency of the dynatron generator

Abstract: This paper develops a method of calculating the effect which the curvature of the characteristic has on the frequency of the dynatron generator. The analysis shows that the fundamental frequency is affected by the phase of the higher-harmonic components of anode potential and that the effect would be zero if the impedance of the circuit could be made purely resistive to all the higher-harmonic currents. A network is described which fulfils closely the required conditions. It is found that the change of frequency which accompanies the change of output is much greater than can be accounted for by the curvature of the characteristic, and hence it is presumed there must be some other cause of instability inherent to the valve. It is suggested that the electrode capacitance may depend on the condition of the space charge.

Frequency-changing at supply frequencies by static means

Abstract: The theory of the static frequency-changer is developed and a number of expressions of a simple character are obtained which enable the performance to be calculated for any loading conditions.It is shown that, knowing the magnetization characteristic of the iron, the maximum attainable output per unit volume of iron for unity-power-factor conditions in the load may be calculated for the ideal transformer from a simple formula.It is found theoretically that if the load is of suitable impedance and leading power factor an output from a given weight of iron is obtainable which is many times that obtainable when the load is of unity power factor. The power factor of the input is also improved.Experimental work, confirming the theoretical work, is described.Finally, design particulars are given of a 550-kW unit, changing from 50 cycles to 150 cycles with an efficiency of 94 per cent and output power factor of 0–53 leading. The input power factor of the unit as designed is 0–38 lagging, but this may be increased by the use of additional capacitative kVA on the higher-frequency side.

The steel-melting coreless induction furnace

Abstract: The paper aims at giving a survey of the properties, characteristics, and position, of the electric induction furnace as used for steel-melting. Mathematical considerations are not given full, but sufficient are included to show the extent to which the skin effect is of importance. A 1/4-ton furnace is described, together with some details of the auxiliary apparatus and the usual lay-out in steelworks.The performance of such a furnace is indicated, with typical wattmeter traces of runs; and an attempt is made to show the manner of the energy dissipation, thus affording an insight into the efficiency of induction heating.Finally, some consideration is given to the metallurgical aspect of the furnace, with a description of the Rohn method of making a lining. An extension of the usefulness of the type of melting unit is urged and its potentialities for refining are outlined.

An analysis of the costs of electricity supply and distribution in Great Britain, with some suggestions as to the causes of and remedies for the slow rate of development

Abstract: The capital invested in electricity supply during the 10-year period 1921–22 to 1930–31 is allocated as between generation and distribution, showing how the latter is becoming the more important side of the industry.The revenue derived from sales of electricity during the same period is also reviewed and allocated as between generation and distribution expenses, showing that although the efficiency of generation has increased materially, there has been little, if any, improvement in distribution efficiency. The causes of this lack of improvement are analysed andsuggestions are put forward for the co-ordinated control of distribution in order to effect improvements and economies.

A Radio-Frequency Bridge for Impedance and Power-Factor Measurments

Abstract: The paper deals with the problems which arose in the adaptation of the Schering bridge network for service at radio frequencies, and describes the final form taken by the arrangement of components, screened high-frequency source, and screened detector-amplifier, which has given a satisfactory performance at frequencies as high as 1 million cycles per sec. The steps which had to be taken to ensure a simultaneity of the main and the auxiliary bridge balances and to expedite the convergence of the succession of balances upon the final simultaneous balance are explained.An account is given of the modifications which have been introduced into the well-known Schering bridge network, and of the provisions which had to be made in the disposition and the linking-up of the component parts to render the bridge capable of precision measurements.The arrangement described is essentially an equal-arm bridge composed of two capacitance arms, two resistance ratio-arms, and two auxiliary resistance ratio-arms, the latter forming part of a Wagner earthing system. The resistance arms are provided with condensers in parallel for phase-angle adjustments. Tappings are brought out in the main arms which permit of a choice of resistance for the whole arm and for the fraction across which the condensers are shunted. The artifice of connecting the power-factor- measuring condenser across only a fraction of the main resistance was adopted in order to supply a means of maintaining the changes of capacitance which are required when dealing with a large variety of power-factor differences over a large range of frequencies, always of the same convenient order of magnitude.The procedure followed in the measurements is strictly one of substitution of a variable air condenser for an actual, or an effective, capacitance of approximately the same value. Apparatus such as coils are tested in series or in parallel with such a value of the capacitance in the arm as brings the effective capacitance of the composite arm to the value required for a bridge balance. Observations are taken of the change in reading of the standard air condenser and of the difference in setting of one of the power-factor-measuring condensers required by the inclusion and the exclusion of the impedance under test. From these two differences the two components of the impedance may be computed. In the case of condensers, the condenser is replaced by the variable air condenser. The capacitance is obtained directly from the settings of the latter. The power factor is derived from the formulaΔ θ = 1/ρ2R Δ C ω KT/KSwhere R is the resistance of the ratio-arm, and 1/ρ is the fraction across which the difference δC is observed. KT represents the total capacitance forming the arm, and KS the substituted capacitance under test. The corrections to the simpler formula, when the ratio-arms and the arm under measurement possess large phase angles, are discussed and evaluated. In order to reduce the phase angles of the resistance arms to a minimum, the latter have been provided with a shield, independently earthed, in preference to a separate screen connected to their common point.The successful performance of the arrangement is to be attributed in the first place to the attention which has been given to the design of the high-frequency screened source and of the detector-amplifier, and secondly to the arrangement of the component parts of the network and of the system of wiring which has been developed. The adoption of toroidal forms for the tuning coils of the source and of the high-frequency stage of the amplifier, together with the screening arrangements employed, has practically eliminated all stray inductive and any capacitative coupling between the source and the bridge arms or the amplifier, respectively. Reversal of the leads from the source at 1 million cycles per sec. affects the power-factor balance by only 0.000002.In approaching the final balanced state of the bridge, the usual procedure is followed of adjusting two vectors in the main network for a balance of the main bridge, alternately with the adjustment of two vectors in the Wagner earthing system for balance of the auxiliary Wagner bridge. In practice a large number of alternations have to be made. The fewer that are necessary, the more convergent is the balancing process. The entry of parasitic voltages into the detector system (which is inconsistent with a condition of simultaneous balance of both systems) was found to retard the process very seriously, and when the impedances composing the bridge arms exceeded a certain value the process even became divergent. Consequently, the detector points of the main bridge, and the Wagner bridge, respectively, have been brought close together, and the whole detector circuit has been so designed as to contain no open loops. All exposed points in the amplifier have also been carefully screened. In order to ensure satisfactory conditions for convergence it became necessary to connect the filament of the input stage of the amplifier to the detector point located at the junction of the capacitance-arms. The grid is therefore transferred between the two remaining detector points in the course of the balancing alternations. It was discovered that the convergence could be regulated and further improved by the suitable choice of a resistance which is inserted across these detector points at the amplifier end. The bridge thereupon became easily workable, and the balance was found to be almost independent of the values of the impedances connecting the detector points to earth. The potentials of all three detector points are therefore very close to earth potential.In its present form it is considered that the bridge arrangement is capable of measuring, to an accuracy of 1 per cent, power factors lying between the limits 0.001 and 0.1, and differences in power factor of precision air condensers to an accuracy of 0.00001 at all frequencies up to 1 million cycles per sec.The paper concludes with a list of a.c. measurements for which the bridge is recommended.A solution of the general bridge equations, taking account of impedances from all four corners of the network to earth, is included as an appendix.

Ionization in cable dielectrics

Abstract: The phenomenon of ionization in the dielectric has been selected for detailed discussion in this paper, as it lies at the root of success or failure in the operation of high-voltage cables. The two leading features of the paper are a fairly complete co-ordination and critical examination of the published work on the subject and an account of some of the author's own investigations made with the object of confirming existing suggestions and extending our knowledge along certain lines to still wider limits. As ionization is usually-demonstrated by its effects on the variation of power factor with voltage, several pages are devoted to different phases of this important relationship, and the physical mechanism controlling the appearance or modification of ionization is then reviewed and illustrated. The close connection between ionization and long-time breakdown strength receives some attention, and tentative suggestions are made for the assessment of quality. A noteworthy feature of the paper are the data demonstrating the advance which has been made in applying a knowledge of the phenomenon of ionization to the production of reliable solid-type high-voltage cable.

Synthetic corundum for jewel bearings

Abstract: The paper gives a general description of the manufacturing processes used in the production of rough synthetic corundum and of jewel bearings for electricity meters and other measuring instruments.In Part 1 of the paper typical characteristics are illustrated which are peculiar to natural and synthetic sapphires, to show to what extent the lack of homogeneity of the material can impair its qualities as a bearing.Part 2 deals with the production of synthetic rough sapphires. Reference is made to the method and the apparatus now employed for the synthesis of sapphire.In Part 3 the manufacture and design of jewel bearings is described in a general way, and some special characteristics of corundum are given.

Some characteristics of short-wave propagation

Abstract: In Part 1 of the paper is given a description of the phenomena observed when using a cathode-ray direction-finder on closed coils for examining received signals on frequencies of the order of 10 000 kilocycles per sec. The two outstanding features are the systematic appearance of certain cyclic forms, and the large values obtained for the horizontally polarized electric components.The former are then examined in the light of the magnetoionic theory, and it is shown that the majority of them admit of simple explanation on this basis. The latter are chiefly of interest in raising the question of angle of incidence, which appears to be much less than is generally supposed; and Part 2 is devoted to a more detailed examination of the question.

The relative fuel economy of electricity, gas, oil, and solid fuel, as heating agents

Abstract: The paperis an effort toexplain the reason why, and the conditions in which, electrical power can be used competitively for the heating of buildings, in spite of the fact that the cost of the crude heat is several times greater than that of any other heating medium. It deals exclusively with the cost of the fuel and the labour inevitablyincurred in handling it, and not with questions of capital cost and allied matters. It details the causes of loss of heat inevitably incurred in the use of any other form of fuel, estimates their respective amounts, and traces the stages by which the heat generated at a central boiler-house is frittered away on its way to the rooms where it is required. The principal features which enable electrical power whensuitably installed to compete withother fuels are its 100 per cent technical efficiency, its instantaneous control, and the very low time-lag and absorption. It is shown that, owing mainly to these features, an amount of heat fully equivalent to the energy of the electrical current used can be delivered at the exact time only when it is required, thereby avoiding the waste caused essentially by the lack of accurate timing inevitably associated with all systems in which heat is generated in and distributed from acentral boiler, whether or not this iselectrically heated. In this sense electricity, locally applied, is the only means by which an overall ?efficiency? of 100 per cent can be rationally claimed for an entireinstallation?in the sense thatthe amount of electrical energy used isequal to the bare minimum of theheat physically necessary to secure the desired result. The overall efficiency of the heating process by other media is determined as thecontinuous product of numerousdetailed efficiencies (corresponding tothe various stages of loss), each of which is so defined that their continuous product may be equal to a final ?overallefficiency? representingthe ultimate ratio between the heat physicallynecessary for securing the desired result and that in the fuel purchased. Representative maximum, medium, and minimum values are in all cases given of each of these factors where they areliable to wide variation, and the tables are drawn up in such a form that they can readily be corrected for any variation in these assumed values. A close comparison ismade between the features of electricityand gas (herein called the ?mobile fuels?) as heating agents; also between the operating characteristics of the ?portable fuels??oil and coke. The labour involved in handling thefuel in each case is considered in detail. A sharp distinction is drawn between the relative costs in the respective cases of continuous and intermittent heating. It is inthe latter that the economy of electric heating showsto greatest advantage; the reasons for this are given. Explanationsare advanced of the reasons why instantaneous control, suchas is only obtainable by electricity, is of great importance in restricting the unnecessary use of heat and at the same time in supplying an adequate amount for main taining comfort conditions during all times of actualoccupation, and onlyduring those times. Numericaltables and diagrams are given showing theprogressive loss of heat in the various stages through which the heat passes, and explanations are given of the details of thecalculations of the heat quantities really necessary for maintaining the required temperatures, as well as of the proportions inevitably dissipated in the process of delivering heat from a central point to the rooms. The ratio between these iscalled the ?occupation efficiency?, the incidence of which on the calculated result is fully explained. Finally a table is given in which, assuming fixed prices for the crude heat, average medium values for the various factors involved and theoretical perfection of the heating result for an assumed period, the relative annual costs of heating 1 000 cubic feet of average space by the various media is calculated. It is emphasized that all the factors on which these costs depend are essentially very variable and that no such table of relative costs can conceivably be absolute apart from the factors involved.

Mutual induction between power and telephone linesunder transient conditions

Abstract: One of the most important problems in telephone interference at the present time is that of the dangerous voltages induced in communication circuits when an earth fault occurs on an adjacent high-tension transmission line with earthed neutral. Theoretical formulae have been developed which enable the coupling between the two circuits to be calculated, and measurements in the field have shown that, provided the resistivity of the earth is known, these formulae will enable an accurate forecast to be made of the induced voltage in the communication line, after steady fault con ditions have been established. The present paper examines the derivation of a mathematical expression for the coupling under the initial transient conditions. In order to see whether the variation between the initial and final values of the coupling is important, a number of practical cases have been worked out numerically for the first time. This has enabled curves to be drawn showing the induced voltage during the first few microseconds of a fault. The result of the investigation is to establish the sufficiency of the simpler steady-state methods of dealing with the problem for practical cases.

The inter-electrode capacitance of the dynatron, with special reference to the frequency stability of the dynatron generator

Abstract: The work described in this paper was undertaken in the Engineering Laboratory at Oxford University, for the purpose of investigating the cause of the frequency changes which occur when the operating voltages of a dynatron generator are altered.The changes of frequency are such as to indicate that the anode-grid capacitance is a function of the anode potential and is independent of the parameters of the oscillating circuit. The same changes of capacitance are found to occur when the circuit is not oscillating and the valve is used to reduce the power factor of the circuit. The dependence of capacitance on space charge is discussed briefly and outlined analytically. The dynatron characteristic is then analysed and the probable form of the secondary emission-current/anode-potential curve is deduced. This deduced curve indicates the presence of space charge in the operating conditions where the changes of frequency are found to occur.

Electric lighting of buildings

Abstract: Part 1 of the paper emphasizes the relationship between modern lighting design and contemporary practice in architecture and decoration, due to the use of new constructionalmethods and materials. Various forms oflighting are described briefly, and instances are given, leading up to the more recent influence that lighting by electricity is bringing to bear on modern architectural form. Part 2 deals with the characteristics of modern electric light sources and of the materials used in modern lighting, particularly diffusing glass of various kinds. The visual factors entering into lighting problems are then discussed, with particular reference toglare and its avoidance. The necessity for adequate contrasts, the proper utilization of shadows, and the importance of colour in lighting, are dealt with in the concludingparagraphs of the paper.

Destructive voltage-rises on low-tension busbars. A suggestion

Abstract: The paper treats of a class of low-tension ac distribution system using bare conductors for electrical flexibilityA formula for self-inductance is calculated, from which it is shown that, given certain limiting conditions, the conductors must be unusually closely spacedIn practice, such a system is subject to frequent and apparently inexplicable flash-oversInvestigating this problem, the paper analyses mathematically the effect, on a steep-fronted voltage surge, of regularly localized insulator capacitanceThe wave-complex arising at each reflection point is found to be divisible into groups, provisionally termed “sub-transients,” which are expressible by gamma functionsIt is concluded that the insulators have a magnifying effect on voltage surges, which thereby easily attain destructive valuesThe researches of Steinmetz confirm the tendency for the phenomenon to persist, in systems of the type consideredInsulating barriers are suggested as being a proven, if expensive, remedy

The single side-band system applied to short-wave telephone links

Abstract: In Part I it is pointed out that only a few of the existing 50 radio-telephone links are at present working at anything like full-load capacity. For this reason the chief problem for the engineers on most of the links is one of reducing operating costs. On a few heavily-loaded links, however, the expense of some improvement in circuit performance would be justified. Various means are discussed in outline for dealing with these two problems. In Part II further details of the improvements proposed in Part I are dealt with, and in particular the probable value of the single side-band method in relation to other possible solutions is discussed. (1) Operating costs.?It is found that on an existing typical link, with either light or heavy traffic, the single side-band suppressed-carrier system would be expected to save about 87 per cent of the valve replacement costs, and 90 per cent of the cost of the power. (2) Improved performance.?In the cost estimates of (1) it is shown that changing to single side-band working should be equivalent in signal/noise ratio to increasing the peak power of the transmitter about 16 times. If this factor is allowed for, by greatly reducing the transmitter size, it is shown that single side-band working should still give circuit improvements in relation to fading, quality, and privacy. The single side-band system eliminates the chief cause of distortion at present, i.e. that due to selective fading-out of the carrier. It should also make workable several well-known privacy schemes at present useless during selective fading, and give rise to a promising new method. A possibly new means of reducing distortion due to intermodulation in Class 3 single side-band amplifiers is referred to. In Part III an attempt is made to answer the following three questions:? Is the problem technically possible at all under present commercial conditions ? If it seems possible, what is likely to be the best general method?independent stable oscillators at each end, or the use of a pilot signal ? Having decided on the most promising general method, what problems will be met in designing practical equipment, and how can we solve these problems ? The answer to the first is in the affirmative; single sideband working should be possible in practice, either by separate stable oscillators or by a pilot signal. A discussion of the second question results in favour of the pilot method, particularly as it is shown that some sort of pilot is almost indispensable in any case, for controlling the medium-period fading. Answering the third, it is suggested that a single radio frequency corresponding to an audio frequency of about 3.4 kilocycles per sec. would be most suitable, and that this pilot should control the frequency of a local oscillator. It is pointed out that exact synchronizing in the correct phase would in any case be useless at present; while selective fading is still not overcome, the exact signal wave-form can never be reproduced at the receiver, even with perfect carrier synchronism. Experiments were carried out by the author on three shortwave links using equipment on the lines suggested in Part III, and the results are given in Part IV. The final tests between Madrid and Paris showed that the expected improvements over double side-band working, discussed in Part II, were fully obtained. A maximum synchronizing error of 6 cycles per sec. over periods of several hours was found. In Part V it is suggested that? (1) A simplified form of the receiver described in Part IV could be used commercially, and would meet requirements for at least the next few years. In this connection a some-what new form of filter, on the ?balanced reaction? principle, is described. (2) For the immediate future the principle used in the transmitter should be that of the ?side-band balance,? i.e. a method of selecting the side-band in one stage directly at the final high frequency, without relying on filters. This circuit is discussed. Assuming selective fading to be absent, it is shown in Part VI that synchronizing at the exact frequency, within predetermined phase limits, is fairly simple. One form of circuit to do this is described. A high-quality circuit is explained, giving exact synchronism in phase except during periods of deep fading, when the frequency difference may amount to a few cycles per second.

The characteristics of submarine telephone cables at carrier frequencies

Abstract: This paper deals with the electrical characteristics of submarine cables designed to transmit carrier frequencies. The sphere of this branch of communication is outlined, and the general principles of operation and cable design are stated.The use of bridge networks to determine the characteristics of short lengths of cable, and of a completed laid cable, is described. Test data obtained during the manufacture of a carrier-frequency cable are given, and predictions made therefrom are compared with the results of impedance and attenuation tests on the laid cable. Alternative methods of measuring average and high attenuations are specified.In view of the probable future extension of this form of communication, recent improvements in materials and their effect on transmission capacity are considered. The chief improvements are in new dielectrics having considerably less leakance than those commonly used in submarine cables, and in magnetic loading material adaptable to the carrier-frequency range.

A resistor for high-voltage measurements

Abstract: A cheap and compact 100-megohm resistor for the measurement of direct or alternating voltages of power frequency may be constructed from thin-film resistance units similar to those commonly used as “grid leaks”. The construction and performance of such a resistor are described. Errors due to corona or leakage along supports appear to be absent. There is a change of resistance with loading, due to the self-heating of the resistor. The magnitude of this change is approximately proportional to the square of the applied voltage and amounts to −2.6 per cent at 100 kV. By taking this change into account, voltages up to 100 kV may be measured correctly to 1 or 2 parts in 1000.

Some electrical methods of remote indication

Abstract: Indication of a measured quantity at a distance from the point where the measurement is carried out has become of great importance, in view of the modern practice of centralized control. At the control point, indications must be given of all quantities necessary for the efficient operation of the system, and thus remote indication is essential. This paper deals with the principles on which some electrical methods of remote indications are based. Numerous systems have been developed, in all parts of the world, and for purposes of description they may be classified according to the fundamental principles on which they are based. A description is given of at least one system in each class indicating various applications of the general principle of the class. The conditions under which an equipment is to be used, and the characteristics of the various systems, will determine which of the numerous methods described should be chosen for a particular application.

The heating of domestic pendant lamp fittings and their connecting leads

Abstract: This paper is an investigation of the temperature-rise of the rubber insulation of leads connected to pendant fittings using lamps up to 200 watts as now made for domestic use, and of methods for reducing existing values to safe limits On account of the higher filament temperature of the gas-filled lamp than the vacuum lamp, and the presence of the gas in the bdlb, the temprature of the lamp cap and of the surface of the glass is much higher in the former case than in the latter This causes the connecting leads to become unduly hot and their deterioration to be considerably accelerated As a result, after a limited period of service there is a risk of short-circuit or burning of the flexible cord occurring adjacent to or within the holder There is the further possibility of metallic holders becoming alive Although these risks exist, it is by no means common for serious trouble to occur Four general methods are available for mitigating these effects, viz:- (i) By improving the heat-resisting properties of the rubber insulation (ii) By replacing rubber by a material capable of withstanding, existing working temperatures (iii) By modifying the design of fitting and lampholder so as to reduce the working temperature of the rubber to 50°C (iv) By modifying the design of the lamp so as to reduce the working temperature of the rubber to 50°C Methods (I) and (ii) have both been under consideration by the Cable Makers' Association, and as a result the ERA has been advised, since the preparation of this paper, that in regard to method (i) it would be possible to supply an improved type of rubber-insulated flexible cord which would give satisfactory operation at 60°d C for a reasonable life under reasonable usage; and, in respect to (ii), flexible cords of this description are available and are being used in dry situations This paper is based on the 50° C limit, but the modifications necessary to fittings, etc