Showing papers in "Journal of the Institution of Electrical Engineers in 1951"

The deterioration and breakdown of dielectrics resulting from internal discharges

Abstract: The deterioration of polythene and other dielectrics, when exposed to internal discharges, has been examined under controlled conditions. The discharge-inception voltage and the magnitude and energy of individual discharges have been determined, and the location and number of discharges in voids of different dimensions have been studied, by electrophotography. The discharge sequence is explained by the difference in the distribution of the residual charges on the positive and negative surfaces of a void after a discharge. Accelerated deterioration tests using a frequency of 150 kc/s show that, initially, discharges cause slow erosion at the surfaces of the void and the formation of a transparent film of resin which fluoresces under ultra-violet irradiation. About 10 -15 cm 3 of polythene is eroded by each discharge, probably as a result of thermal degradation. The rate of deterioration increases rapidly with increasing voltage; at twice the discharge-inception voltage, the discharges concentrate and form several deep uncarbonized pits near the periphery of the void. When the pits attain a critical length, the mechanism of deterioration is believed to change; narrow semi-carbonized channels are found and, generally, breakdown follows immediately. Deterioration is greater when the void is adjacent to an electrode than it is when the void is enclosed in polythene. Tests show that, under equivalent conditions, polytetrafluorethylene and perspex are less resistant to discharges than is polythene.

The use of saturable reactors as discharge devices for pulse generators

Abstract: The subject of recurrent high-power pulse generation may be considered as including pulse generators for radar modulation, nuclear-particle acceleration, impulse testing, and auxiliary and ancillary circuits such as initiators and sub-modulators for more powerful discharge devices, e.g. initiators igniter firing-circuits. In generators of these types it has been customary to use electronic discharge devices which depend for their operation on the conducting properties of electric arcs as the means of rapidly discharging capacitive pulse-forming networks into appropriate utilization circuits. While these devices are, in general, satisfactory for many applications, it is well known that they have inherent disadvantages. Comparatively recent developments in the field of high-permeability high-saturation-flux-density magnetic materials having characteristically rectangular hysteresis loops, together with new circuits designed to take advantage of their special properties, have made it possible to overcome many of the disadvantages of electronic discharge devices by replacing them with static components having indefinitely long lives. The paper outlines the historical background to the development of some of the circuits and materials and describes the operation of circuits designed to fulfil the duties of radar pulse-modulators and ignitron firing circuits. Some practical and theoretical considerations affecting the design of the special components peculiar to the work are also described.

A history of the theory of information

Abstract: The paper mentions first some essential points about the early development of languages, codes and symbolism, picking out those fundamental points in human communication which have recently been summarized by precise mathematical theory. A survey of telegraphy and telephony development leads to the need for “economy,” which has given rise to various systems of signal compression. Hartley's early theory of communication is summarized, and Gabor's theory of signal structure is described.Modern statistical theory of Wiener and Shannon, by which “information” may be expressed quantitatively, is shown to be a logical extension of Hartley's work. A Section on calculating machines and brains attempts to clear up popular misunderstandings and to separate definite accomplishments in mechanization of thought processes from mere myths.Finally, a generalization of the work over the whole field of scientific observation is shown, and evidence which supports the view that “information plus entropy is an important invariant of a physical system” is included.

The ionospheric propagation of low- and very-low-frequency radio waves over distances less than 1000 km

Abstract: In the last few years a large part of the radio research at the Cavendish Laboratory has been concerned with the propagation of waves of low and very low frequency. The paper constitutes a summary of the results of the various experiments, which are described in detail in separate papers, some of which are as yet unpublished.3?10 The results of various independent methods of measuring the apparent height of reflection of the waves show that waves of 16?30 kc/s are reflected as if from a sharply bounded horizontal surface situated at a height of (72 ± 3) km when the sun is overhead. The apparent height of reflection varies regularly with the angle of the sun and its variation may be summarized by an equation. The waves of frequency 30?150 kc/s appear to be reflected from a height of about 75 km at oblique incidence, but there is some evidence that they may be reflected from as much as 10 km higher at vertical incidence. The polarization of the waves at all frequencies is found to be approximately circular at steep incidence, but at oblique incidence (65 ) waves of a frequency of 16 kc/s are linearly polarized. No measurements of polarization at oblique incidence have been made on the higher frequencies. The absorption of the waves changes very rapidly with frequency?on a summer day the conversion coefficient varies from about 0.15 at 16 kc/s to 0.002 at 70 kc/s. Important differences in behaviour near sunrise are observed on all frequencies at steep incidence and oblique incidence. The effects of a sudden ionospheric disturbance on the reflected waves are discussed and interpreted as implying a decrease in the apparent height of reflection; the amplitude of the reflected wave is scarcely altered on 16 kc/s, but is much decreased on higher frequencies. Finally, the present state of the theory of reflection of very long radio waves is discussed very briefly.

The theory of parallel-plate media for microwave lenses

Abstract: The problem of the reflection and refraction of an electromagnetic plane wave at a plane interface formed by the edges of an infinite set of equidistant parallel metal plates has been studied, the plates being assumed to have no thickness and perfect conductivity. The direction of propagation of the incident wave is assumed to be at right angles to the edges of the metallic plates, and the electric vector is assumed to be parallel to them. The solution presented extends the scope of those already published, in that no restriction has been placed on the number of waves propagated away from the interface in the homogeneous medium; the reciprocal case of a wave incident from the plate medium has also been treated. Further, the mathematical method is simpler than that used in previous solutions. From the formulae derived, curves have been computed of the power-transmission and power-reflection coefficients, together with those of the phase changes across the interface. The formulae have also been tentatively applied to certain lenses, and the resulting curves indicate practical limits to the design of efficient lenses.

Reflection and refraction of microwaves at a set of parallel metallic plates

Abstract: The paper deals with the theory of the propagation of microwaves, passing from a homogeneous and isotropic medium into a set of parallel metallic plates. The plane of separation between the set of plates and the homogeneous medium outside is assumed perpendicular to the plates. The incident wave is plane, and its direction of propagation is in the plane containing the perpendiculars to the plates and to the surface of separation. The electric field is parallel to the plates. The method is relatively simple. The study is based on Maxwell's equations and on periodicity and continuity considerations. An analytic method is introduced for the solution of the infinite set of linear equations with an infinity of unknowns, which arises from the continuity conditions at the boundary plane of the plate edges. Formulae are obtained giving the nature, direction, phase and amplitude of the reflected and transmitted waves. The reflected waves follow the usual laws found in the grating theory. In the case when only one non-evanescent plane wave is reflected, the transmission and reflection power coefficients are those obtained at the junction of two semi-infinite transmission lines corresponding to the free-space and the plate medium respectively. The analogy with transmission lines is more complex when two non-evanescent waves are reflected. For the usual angles of incidence and plate spacings the transmission power coefficient is high (maximum 99.5%), and the phase shift on transmission is small.

Universal high-speed digital computers: a small-scale experimental machine

Abstract: An experimental electronic computing machine has been constructed, using the serial binary-digital system of number representation. The principle of its operation is explained by means of an analogy; and its design and construction are described in detail. Although the machine is small in size, it has been designed with a view to expansion, and will then be intrinsically capable of performing any computation automatically.

Diversity Effects in Spaced-Aerial Reception of Ionospheric Waves

Abstract: A theory is given of the diversity effects produced in spaced aerials by incident waves having a distribution in direction of arrival. The general case of a continuous coplanar distribution of rays with random phase is considered, and when this distribution is narrow and symmetrical it is shown how its extent can be estimated from observations of either the amplitudes or the phase difference of the signals in the spaced aerials. The case of a strong steady signal superimposed on the centre of the continuous distribution is also analysed and the results are obtained in terms of a parameter which may be called the signal/noise ratio. It is shown how, by making measurements of the diversity effect in both amplitude and phase, both this ratio and the angular spread of the continuous distribution can be obtained.Some daytime measurements at nearly vertical incidence have been made on first-order ionospheric reflections at frequencies between 4 and 7 Mc/s, and it is found that the diversity effects observed are most satisfactorily explained in terms of a steady “signal” (or specularly reflected component) superimposed on a “noise” background (composed of the continuous angular distribution due to diffractive reflection). Values of signal/noise ratio of about two or three have been obtained, with a noise background which has an angular spread with standard deviation of about 1°.

Transient theory of synchronous generators connected to power systems

Abstract: Practical methods of predetermining the transient performance of synchronous generators have in the past been based more on general considerations than on a strictly logical theory. On the other hand, general equations for the synchronous machine were derived by R. H. Park and are well known, but the equations have not been widely used in practice because a direct solution is laborious. The paper shows how, by making a number of approximations, simple solutions of the equations can be obtained for short-circuit and other fault conditions, both for the generator by itself and when it is operating with an external power system, assuming the circuits to remain balanced. The formulae obtained for the transient reactances and time-constants agree, in the main, with those deduced from general considerations. The present treatment, however, demonstrates clearly what are the underlying approximations, and shows what simplifications can be made when some of the components of the transient currents can be neglected.

The effect of electric stress on the life of cables incorporating a polythene dielectric

Abstract: Work on the voltage characteristics of polythene cables has been described in a previous paper; the present paper gives the results of further studies. Factors contributing to the breakdown of polythene cables have been investigated, and tests on cables have been supplemented with a similar series of tests on artificially produced voids. Voltage/life characteristics of cables with solid dielectric and a single non-stranded conductor have been examined, and voltage ratings, based on experimental work, are proposed for alternating and direct stresses.

The measurement of permittivity and power factor of dielectrics at frequencies from 300 to 600 Mc/s

Abstract: The paper describes a resonance method of measuring permittivity and power factor which is essentially a development of the Hartshorn and Ward apparatus, suitable for use at the higher frequencies. The method is one of capacitance variation, and the disc form of sample, the circular plate electrodes and the cylindrical form of micrometer capacitor are retained but reduced in linear dimensions. The capacitive elements are mounted in a re-entrant cavity, and the micrometer capacitor is so placed that the voltage across it is only a fraction of that across the plate electrodes; this increases the fineness of adjustment. The micrometer capacitor can be calibrated only by using the cavity as a wavemeter; the capacitance settings were therefore expressed in terms of the corresponding resonance frequencies, and the calculations of power factor then became identical in form with those of the frequency-change method, although the measurements were made at a constant frequency. After applying the appropriate corrections the permittivity can be determined to within ± 1% over most of the range. The high Q-factor of the cavity and the precision of adjustment of the micrometer capacitor make the power-factor determinations highly accurate. For values less than about 0.01 the error is estimated as being less than ± 2 × 10−5 (for measurements on solids). For higher values a superheterodyne receiver incorporating a piston attenuator is used as the detector, and the estimated limits of error for such measurements are ± 5 × 10−4.

The development and design of high-voltage impulse generators

Abstract: The paper explains the need for high-voltage impulse generators which will produce the standard voltage waveshapes specified for impulse testing. A theoretical analysis of the impulse generator and load circuit is given, and the development of the multi-stage generator for high voltages is described. The mechanism of operation of the multi-stage generator is analysed and is illustrated by oscillograms. Illustrations of two typical generators are shown, and some of the auxiliary equipment needed is briefly mentioned. The paper concludes with an example of impulse-testing technique.

A phase-comparison method of measuring the direction of arrival of ionospheric radio waves

Abstract: The most convenient research method of measuring completely the direction of arrival of waves reflected at the ionosphere appears to be that in which the phase differences of the signals received in an assembly of aerials are measured. If two pairs of similar aerials erected on lines perpendicular to each other are used, two independent phase angles may be obtained from which both angle of elevation and azimuth may be deduced. The apparatus described uses spaced coaxial loop aerials at a separation of 100 m. The signals from the aerials in a pair are amplified by means of matched receivers. The phase difference between the output signals from these receivers is displayed direct on a cathode-ray tube as the angle of inclination of the trace. With pulsed signals emitted from a suitable transmitter and with corresponding timing equipment in the receiver, the individual rays making up the total ionospheric signal may be separated from each other. The apparatus covers the frequency band 4?15 Mc/s, and the r.m.s. error of phase measurement is about 1°. Site errors, however, set a more severe limit to the accuracy of the directional measurements than do instrumental errors, and in practice it is found that, for example, over an oblique path corresponding to a range of 700 km, bearings can be measured with an accuracy of about 1° while the angle of elevation can be measured with an accuracy better than about 1?° so long as it exceeds 30°. These limitations mean that angles of elevation of E-layer reflections cannot be measured accurately at long range; it is possible, however, to obtain measurements of useful accuracy of the angle of elevation of F-layer reflections at ranges up to 1000 km or more. Bearings can be measured accurately at all ranges and for all reflections. The apparatus has so far been used principally for the study of F-layer reflections.

The Intrinsic Electric Strength of Polyvinyl Alcohol and its Temperature Variation

Abstract: The intrinsic electric strength of polyvinyl alcohol has been measured at temperatures ranging from -195° C to + 90° C. At the lowest temperature, the electric strength is slightly higher than that of any other material yet measured, but it falls rapidly with increasing temperature throughout the range. Comparison of this behaviour with that of other high polymers suggests that the temperature variation of the intrinsic electric strength of these materials is determined by the dipoles present, and not by the physical structure. Lack of information on the value of the dielectric constant at low temperatures precludes exact comparison between the actual electric strength in this region and values calculated from Frohlich's formulae, but satisfactory agreement was obtained between the experimental and calculated values for polyvinyl alcohol, using the high-frequency value of the dielectric constant.

The influence of rectifier harmonics in a railway system on the dielectric stability of 33-kV cables

Abstract: The electrified sections of the Southern Region of British Railways outside the London suburban area derive their traction current requirements from mercury-arc-rectifier substations, which, in turn, receive high-voltage power supplies from the British Grid by means of a 33-kV railway-cable network. The earliest installation was that between Croydon and Three Bridges, which was brought into operation in July 1932; satisfactory operation was experienced until 1944, when it became apparent that the incidence of 33-kV cable failures had risen. The paper shows that surges were not responsible for these failures, but that the harmonics generated by the rectifier equipment, together with the system resonance characteristics, were such that the waveforms of the system voltage were considerably distorted. The conditions were aggravated by irregular operation of the rectifiers. It is deduced that this waveform distortion is mainly responsible for shortening the life of the dielectric material of the impregnated supply cable. An unusual feature of the cable damage is the presence of a number of uncarbonized pinholes in the paper tapes in the vicinity of the fault; it is concluded that these pinholes are caused by internal discharges, accentuated by the presence of the relatively high-frequency and highamplitude harmonic voltages in the system. An explanation is offered to show why the greater proportion of the cable failures occurred in sections where the cable was either in the ground or in a tunnel. The paper concludes with a short discussion of the practical methods of reducing or avoiding similar troubles.

An accurate logarithmic counting-rate meter covering a wide range

Abstract: The paper describes an instrument whose output voltage is proportional to the logarithm of the rate at which the input pulses are received. This relationship can be made to hold, to a good approximation, over a range of pulse rates of from 1 to 105 pulses per second without range switching.The causes of inaccuracy and instability are examined, and it is shown that errors may be expected to be less than 10% of the pulse rate throughout the range of a five-decade instrument; the errors are correspondingly smaller in an instrument covering a smaller range.The paper shows how the instrument may be used in the investigation of some problems in nuclear physics.

The determination of time and frequency

Abstract: In the measurement of frequency to a precision of 1 part in 108, or better, it is necessary to consider small variations that may exist in the unit of time. The determination of time is an astronomical process, and the paper reviews briefly the principles involved. Various factors affecting the uniformity of the observed time are described. Reference is made to the astronomical instruments employed or planned for use for the determination of time at the Royal Greenwich Observatory. The clocks and other equipment are next described, together with the methods currently employed in the operation of the time service.The standard of accuracy now attained is discussed in relation to both time and frequency, and it is shown that, for frequency, agreement between Greenwich and the U.S. Naval Observatory is normally within 1 part in 108.In practice, two time systems are now employed: Greenwich Mean Time, based directly on the astronomical observations and applicable to surveying and astronavigation, and a more uniform time system which is not yet precisely defined, suitable for accurate work in frequency measurement and in related scientific investigations.

Low-frequency radio-wave propagation by the ionosphere, with particular reference to long-distance navigation

Abstract: The paper discusses radio-wave propagation in the 70-300-kc/s band for long-distance navigational applications. Signals may reach the receiver by the ground path and by reflection from the E-layer of the ionosphere, the resultant signal at the receiver terminals being displaced in phase or time to a degree dependent upon the relative magnitude and phase of the component signals. Time-error curves are shown for typical propagational conditions, both by day and by night, at various frequencies. The errors which occur are classified as follows: (a) Systematic errors for which corrections corresponding to the propagational conditions present at the time can be applied. (b) Random errors which depend upon the difference between the ground- and ionospheric-wave paths. (c) Random errors arising from the imperfections of the ionosphere as a reflecting layer. The characteristics of these propagational errors are discussed, and the magnitude of navigational errors arising therefrom are deduced, using suitable examples. It is shown that, at intermediate distances, the random position-line errors are mainly dependent on the difference in the overground path distance, and that at large distances, they are dependent on the imperfections of the ionosphere as a reflecting layer. From phase and amplitude observations made with receivers in aircraft, during both day and night, the relative values of the ground-and ionosphere-reflected wave components were obtained as a function of distance from the transmitters; from these observations the virtual height of the reflecting layer and the reflection coefficient at oblique incidence were also deduced. The mean height of the reflecting layer is about 70 km in day-time and 90 km at night, with reflection coefficients at oblique incidence of 0.05 and 0.25 respectively. Using the Consol, Decca and Post Office Position Indicator transmitting systems, data collected from fixed receiving points at various distances from the transmitters show

The position synchronization of a rotating drum

Abstract: A servo system is described whereby a rotating drum has its angular speed and angular position controlled to fine limits by using a crystal oscillator as a stable reference. The device was developed for use as a ?magnetic storage system? forming part of a digital computing machine; it has, however, other possible applications. The various components and their functions are described individually and an overall analysis developed. The theoretical performance is compared with that obtained experimentally, and a discussion of the factors affecting the final accuracy is included.

The propagation of metre radio waves beyond the normal horizon. Part 1: Some theoretical considerations, with particular reference to propagation over land

Abstract: The effect on radio wave propagation of departures from the normal variation with height of the refractive index of the lower atmosphere is considered with special reference to frequencies of 90 and 45 Mc/s. The relative importance of changes in refraction near the surface of the earth and of reflection from high-level inversion layers is investigated, and some illustrative examples are worked out. The number of parameters concerned in the problem prevents an all-embracing survey of the many possibilities within the scope of the present work, but a few general conclusions are reached.It is unlikely that radio ducts of sufficient magnitude to be greatly significant at the frequencies in question can be maintained over long distances in broken country. Of the two mechanisms—refraction near the surface of the earth and reflection at an elevated inversion layer—the latter is the one which is likely more often to give abnormally high field strengths at ranges of a few hundred kilometres, especially for relatively low terminal points.Consideration is also given to the effects of scattering of radio energy by turbulent eddies in the atmosphere, and an approximate calculation is made of the magnitude of this scattering for horizontally polarized waves on a frequency of 90 Mc/s, and at distances up to 500 km. It appears likely that such scattering is of less importance in the creation of unusually high field strengths up to distances of the order of 250 km than the mechanisms of abnormal refraction, including reflection at elevated inversion layers.

The development of electrical anemometers

Abstract: The paper outlines the development of instruments for measuring and recording wind speed and direction by various electrical means. Methods of calibration and standardization are discussed in relation to the accuracy and speed of response of standard instruments, and some further models which were made for special investigations, or which are still in the development stage, are described. For general use in the Meteorological Office, the most suitable combination of instruments is a cup anemometer of the generator type with an autographic recorder consisting of some form of recording voltmeter and a remote-recording wind vane connected by a self-synchronous a.c. transmission system to a recorder which is capable of being operated by a small torque. The cup anemometer will record the speed of the wind and the wind vane will record its direction.

Some Adverse Influences of Meteorological Factors on Marine Navigational Radar

Abstract: The absorption of electromagnetic waves in the atmospheric gases, and their absorption and scattering by various forms of precipitation, are discussed from the aspect of their effects on the performance of centimetre-wave radar equipment. Particular attention has been devoted to the phenomena to be expected at the wavelength of 3.2 cm, since this is the wavelength now used, and likely to be more widely used, for marine navigational radar. At a wavelength of 3.2 cm, absorption by the atmospheric gases is hardly significant, and absorption and scattering in rain are likely to cause most of the appreciable reductions in the range of detection. Whilst it is not possible to make a complete generalization, it appears that, at this wavelength, the deterioration in radar performance on targets having echoing areas greater than, say, 2 000 m2 (e.g. ships of more than about 10 000 tons) arises mainly from attenuation produced in rain rather than as a result of masking by echoes from the precipitation itself. With smaller targets, however, this masking will often be the more serious factor. The heavy rates of rainfall common in tropical-equatorial regions will produce a much greater deterioration in radar performance than will the rainfall likely to occur at all frequently in temperate regions. Although very intense snow-storms can produce echoes sufficiently strong to be troublesome, the rate of precipitation required is such that its frequency of occurrence is unlikely to be great. Attenuation occurring in snowfalls is generally of little importance. In dense fogs reductions in detection range may be appreciable?especially in polar regions?when the optical visibility falls below about 25 m; in this case, attenuation is the significant factor.

The vision transmitter for the Sutton Coldfield television station

Abstract: The transmitter described was designed and constructed for the B.B.C. and is the first post-war vision transmitter to be operated on regular broadcasting service in England. The description is general where the technique is well-established, but becomes more detailed concerning novel parts of the equipment.

The measurement of interlaminar resistance of varnish-insulated silicon-steel sheet for large electrical machines

Abstract: In large electrical machines, such as alternators and transformers, that include sheet-steel laminations in their construction, it is essential to insulate the core laminations from one another to reduce eddycurrent stray loss. One method is to varnish the sheets after stamping, before assembly in the core. The need for developing good varnishing technique demands satisfactory measurement of the insulation. One convenient method is to measure the electrical resistance of the insulating film, and this can be done in several ways. The paper attempts to analyse the fundamental problem of the measurement, deduces that it is essentially a statistical problem, and suggests a method by which it may be solved. The paper then proceeds to discuss the possibility of correlation between experimentally measured values of resistance and observed stray losses in completed machines. There is also some consideration of other relevant factors, such as temperature, pressure, and the nature of the plate surface.

The propagation of metre radio waves beyond the normal horizon. Part 2: Experimental investigations at frequencies of 90 and 45 Mc/s

Abstract: Measurements of radio field strength at frequencies of 90 and 45 Mc/s over distances such that direct transmission from the sender to the receiver was impossible are described. Two paths, of lengths 110 and 270 km, have been investigated at 90 Mc/s, and one, of length 160 km, at 45 Mc/s. The statistical distribution of quasi-peak field strengths as a function of time over these paths has been determined, and it is shown that signals considerably in excess of values corresponding to standard atmospheric refraction often occur. An attempt is made to interpret the observed field strengths in terms of the meteorological data obtained from the routine radio-sonde ascents made by the Meteorological Office, and some degree of correlation has been obtained. An indication is given of the bearing of the measurements on the planning of v.h.f. broadcasting and communication services.

The Sutton Coldfield television broadcasting station

Abstract: The paper describes the design, construction and service performance of the Sutton Coldfield television station built by the British Broad-casting Corporation as a first step in the extension of its television service to the provinces.The relation of the operating channel of Sutton Coldfield to the overall frequency-allocation plan for the 41–68-Mc/s band used for television broadcasting in the United Kingdom and the reasons for adopting a vestigial-sideband characteristic for the vision transmission are briefly discussed.The complete vision and sound transmitting equipment is described and its performance characteristics are stated. The considerations governing the design of the aerial-mast system, the vision-and-sound-combining diplexer and the transmission lines are also discussed, and details are given of the construction used and the performance obtained.

Relaxation methods applied to the problem of a.c. magnetization of ferromagnetic laminae

Abstract: The distribution of the magnetizing force inside a ferromagnetic lamination subjected to a.c. magnetization at its surface is determined by the application of Southwell's relaxation methods, on the assumption that the static (or d.c.) characteristics of the material are applicable directly at any point in the lamination under dynamic (or a.c.) conditions. The flux and eddy-current distributions and the ?dynamic hysteresis loop? are then evaluated. The alternative conditions of sinusoidal magnetizing force (at the surface) and sinusoidal total flux are considered for a frequency of 50 c/s. Consideration is also given to higher frequencies of operation when the surface magnetizing force is of sinusoidal waveform.

Some practical equivalent circuits for multi-circuit transformers

Abstract: The exact practical theory of the transformer that has been previously described is extended to transformers with more than two windings. An exact equivalent circuit results which is simpler than that usually used. The equivalent circuit, and the performance of the transformer, are expressed in terms of parameters that can be measured by test. An approximate equivalent circuit is derived in such a way that the influence of the approximation can be expressed in an explicit manner. Some calculations are made of the reactances of three-circuit and four-circuit transformers, in order to indicate how the disposition of the windings affects the performance.

Wind- and gust-measuring instruments developed for a wind-power survey

Abstract: A description is given of two recorders and an anemometer for measuring gusts, which were developed for use in a survey of the available wind-power in Great Britain. The essential feature of the recorders is that they operate unattended for a period of one week. The first instrument is a photographic recorder which shows, at intervals of half an hour, the wind direction and the run of wind past four standard cup contact anemometers. The second instrument records a mark for each two miles of wind, and will make up to four simultaneous records. The anemometer for measuring gusts uses a perforated aluminium sphere as the detecting element and a mechanoelectronic transducer valve to convert the mechanical response to an electrical output. It is compensated for changes of temperature and for the static effects of atmospheric pressure. When used with an amplifier-driven recording voltmeter, the response of the prototype instrument to an instantaneous change of wind speed from 65 to 85 m.p.h., was completed in about 0.07 sec. Modified forms of the instrument are described which measure, respectively, the horizontal components of the wind in two directions at right angles and the vertical component.

Crosstalk in amplitude-modulated time-division-multiplex systems

Abstract: Crosstalk arises in a time-division-multiplex system using amplitude modulation when some of the energy proper to one channel arrives during the time allocated to another channel. Networks inserted in the transmission path will almost always introduce crosstalk, the more so as their bandwidths are increasingly restricted. Analyses of the crosstalk produced by those networks of amplifiers most commonly met with in the transmission path show that no one method of analysis is satisfactory for all networks. Coupling and decoupling networks, producing attenuation distortion at low frequencies, are conveniently tackled by Fourier methods. The use of both networks simultaneously can result in a reduction of crosstalk. The simplest anode loads and those improved by inductance compensation are much better treated by the operational calculus. Experimental results confirm the analyses. Cable problems can also be analysed by operational methods. Because practical low-pass filters introduce serious difficulties in analysis most of the data presented for them are experimental. The oscillatory response found with filters brings new problems. The crosstalk depends on the shape of the transmitted pulse and the duration of the demodulating pulse. Practical systems show little advantage from the use of pulse shapes other than rectangular when the number of networks in the transmission path which distort the pulse shape is small; when the number is large, however, the improvement can be worth-while. The demodulating pulse should not normally be much wider than the transmitted pulse; if it is narrower there may be an increase in crosstalk attenuation.