The Electricity Department of the National Physical Laboratory investigates a wide range of subjects, from the testing of small electric lamps to the effects of lightning on the overhead wires of the Grid. Its work is of incalculable importance to the electrical engineer
THE work of the Engineering Department of the National Physical Laboratory is described in the chapter “Modern Engineering Research”. The work of all the eight main departments of the National Physical Laboratory is of considerable importance to industry, and one of these departments whose research is closely linked with that of the engineer is the Electricity Department.
The theoretical aspects of electricity and engineering are becoming more and more closely allied, since electricity is one of the chief sources of power, of heat and of light. Without it many vast undertakings of the engineer, apart from such works as hydro-electric installations — though their connexion with electrical research may not be obvious to the general public — would be absolutely impossible.
IMPULSE GENERATOR at the National Physical Laboratory. This apparatus, which is capable of developing 1,500,000 volts, consists of a huge bank of condensers which are charged in parallel and discharged in series.
Electrical research covers an extraordinary variety of subjects, from the testing of small electrical instruments and of electric lamps to the investigation of the action of lightning on overhead transmission lines. For this reason the work of the Electricity Department of the N.P.L. is divided into three distinct sections: (1) Electrical Standards and General Electrical Measurements; (2) Electrotechnics; (3). Photometry.
The Electrical Standards division of the Laboratory has charge of the British legal electrical standards — the ampere, the volt and the ohm. The units used in electrical measurements since 1908 are the international units, laid down by an International Conference on Electrical Units and Standards. These units, however, are being displaced by a new system of absolute units expressed in terms of length, mass and time.
The advantage of the new units is that certain troublesome correction factors will be removed from many electrical calculations. In preparation for’ the change, a series of absolute measurements has been in progress at the Laboratory for several years. These measurements have recently been brought to a successful conclusion, and the values of the Laboratory standards are now determined in absolute units.
The values finally accepted will depend on absolute measurements made in other countries, but the general consistency of the measurements made at the N.P.L. provides much valuable evidence for the accuracy of the results.
Although this matter of standards is far from spectacular, by comparison with much of the research carried out at the Laboratory, it is of tremendous importance, and the order of accuracy obtained is extremely high. Certificates for standard resistances, for instance, state the values, within one-thousandth part of one per cent, in terms of the International Ohm.
Standard electrical cells made in accordance with the procedure adopted at the N.P.L. have remained constant to better than one part in 100,000 over a period of twelve years. In the first four or five years the variations are generally no more than two or three parts in a million.
Tests for accuracy of calibration are carried out on condensers, inductance coils, resistance coils, and all types of apparatus used for radio purposes.
Many problems with which the electrical industry is confronted are due to the imperfections of insulating materials, whose behaviour in varying conditions can be predicted with far less certainty than can that of metals. Much work has been done at the Laboratory with the object of assisting industry in this matter. The development of precise methods of testing, the use of these methods, and the cause of those properties of materials which give rise to most of the trouble have all been investigated.
A MILLION- VOLTS SPARK produced in the high-voltage laboratory at the N.P.L. Three transformers, each capable of giving 375,000 volts, can be connected in series for tests of insulating materials at extremely high voltages.
Work of this type finds important applications in the most surprising branches of industry. One example is concerned with the reagents used in the dry-cleaning industry, most of the liquids used being good electrical insulators. For this reason charges developed by friction during the cleaning process are not always able to leak away and accumulated charges may cause a spark and even give rise to an explosion.
The Standards Division also undertakes magnetic tests and research, and such work includes the determination of the properties of permanent magnets, the examination of magnetic steels, the measurement of energy losses in transformer iron and the like.
The Electrotechnics Division undertakes tests of electrical measuring instruments of all kinds — ammeters, voltmeters, wattmeters, supply meters, resistances, shunts and fuses. Work with a more direct bearing on large-scale industry is concerned with the effect of heating and dielectric losses in power cables, the measurement of power loss in insulating materials, and the earthing of electric circuits.
The increasing amounts of power generated in the huge modern power stations necessitate the use of either an increased transmission voltage or an increased transmission current, or possibly both. It is the modern tendency to use the highest practicable voltages for overhead and underground transmission of power, with the object of reducing what might be called the “volume” of the current and therefore the size of the conductor necessary to carry it.
The continued increase in the magnitude of the currents in power stations, however, has raised many new problems. High-voltage cables handling voltages of 60,000 or more are often operated in such conditions that there is a considerable voltage drop along the line. This implies heating of the cable, and the current-carrying capacity of the cables is often limited simply by this rise in temperature, which, if sufficiently pronounced, has a deteriorating effect upon the insulating material.
Since the capital expended in cables , is often of the same order as that required for generating stations, this temperature rise becomes a factor of great economic importance, and its cause and possible methods of reduction are subjects of prolonged investigation.
The rapid development in the transmission of electrical power — especially in connexion with the national distribution scheme —has brought many other problems in its train. A special high-voltage laboratory forms part of the Electrotechnics Division of the N.P.L., and many problems are being studied there. To test the insulators used on the national transmission lines it is necessary to provide a source of high voltage, and the plant used consists of three transformers, each capable of giving 375,000 volts when supplied with power at 1,000 volts.
Impulses at 1,500,000 Volts
These three transformers can be used in series to give a voltage of more than a million. All kinds of insulating material can be tested at these high voltages. Breakdown voltage, effect of immersion in reagents such as water, oils and acids, effect of temperature and the like are all studied.
The high-voltage laboratory, a huge building more than 40 feet in height, is provided at one end with an opening some 30 feet square, through which high-voltage lines may be taken out of doors. A special length of line, suspended from towers of the type normally used for the national transmission lines, is used for all classes of work, particularly those concerned with the effects of lightning and electrical storms on these overhead lines. The million volts transforming equipment is of tremendous dimensions, the largest of the transformers standing some thirty feet high. Specially designed conductors lead to the large spheres across which the discharge leaps.
HIGH-VOLTAGE LABORATORY in the Electrotechnics Division of the N.P.L. In the background are the three transformers used for generating voltages of more than a million. The size of the man standing between two of the transformers gives an excellent impression of the vast proportions of the building.
As the effects of lightning disturbances take the form of sudden surges of voltage with a rapid rise and widely varying rates of fall, it has become necessary to study the “surge characteristics” of insulators and dielectric materials. Means are also being devised for reducing the intensity of disturbances at vital points in generating or transforming stations.
A modern type of lightning arrester depends for its action on a “semiconductor”, whose resistance decreases rapidly as the applied voltage increases. Such an arrester is connected across the apparatus to be protected — possibly at the end of a transmission line — and on the arrival of a high-voltage surge it becomes almost a short-circuit, thus preventing the building up of high surge voltages across the terminals of the apparatus. Arresters designed by different manufacturers are tested at the N.P.L., which has not only the transformer capable of giving an output of a million volts, but also a specially designed impulse generator capable of developing 1,500,000 volts, whose output is claimed to give a faithful reproduction of a natural flash of lightning.
This ingenious apparatus consists of a large bank of condensers, all connected together by a network of high resistances and by a series of spark gaps.
The condensers are charged, in parallel, to voltages of about 100,000, and as soon as a spark occurs across the gaps the condensers discharge in series, the total voltage of the impulse thereby being increased to almost any desired figure. This impulse generator is illustrated at the top of this page.
These high-voltage surge phenomena are investigated with a special cathode ray oscillograph, the tremendous “writing speed” of which makes it possible to obtain a pictorial record of the breakdown of a spark gap with a lag of 0.05 micro-second (one twenty-millionth of a second). With this oscillograph the wave-forms of various types of discharges may be studied visually. The shape of the impulse wave has an important bearing upon the breakdown voltage.
The time-lag characteristics of insulators and gaps, in instances where breakdown occurs beyond the crest of the wave, can be represented by an experimental formula. The important application of this is in connexion with lightning flash-over protectors connected across a string of insulators. The time lag in the breakdown of the protector must be less than that of the insulators which it is designed to protect, to ensure that flash-over takes place across the device. The effect of rain on flash-over phenomena is also studied, and it is not necessary to wait for suitable weather, since artificial rain may easily be provided.
Apart from the work of the high-voltage laboratory, much research is carried out with voltages up to about 100,000, chiefly in connexion with dielectric materials. Cables rated at 11,000 volts, for instance, are tested for their breakdown voltage, which may sometimes be as high as 500,000. Materials such as ebonite are tested at lower figures, and the characteristics of liquid dielectrics such as glycerine and transformer oil are also investigated.
Other important headings under which work has been carried out in the Electrotechnics Division are approval tests of meters for the Electricity Commissioners, the measurement of short time intervals, and tests on cables for marine service, resistance materials for heating circuits, and fuses.
The third section of the Electricity Department of the National Physical Laboratory is concerned with photometry — the measuring of the intensity of light. The Photometry Division was transferred to a new building in 1936, and the importance at the present time of road lighting and industrial lighting in general has been responsible for great activity in this important section of the Laboratory.
INTEGRATING SPHERE PHOTOMETER with a diameter of 10 feet. This instrument is used for photometric tests of high-powered electric lamps. The inside surface is white. It is rendered equally bright all over by successive reflexions, whatever the original distribution of light from the source. This brightness is measured by a photoelectric cell which is screened from the direct radiation of the source of light.
Adequate illumination is of paramount importance in everyday life, and the Photometry Division works in cooperation with H.M. Office of Works and other interested bodies. For several years investigations have been in progress in connexion with the illumination — whether by daylight or artificial light — of factories, offices, art galleries, streets and docks. The effects of glare have led to an investigation of the size of the pupil of the eye in various conditions and, in particular, its rate of expansion after the removal of a glaring source of light from the field of vision. Recent experiments have included the making of a cinematograph film of the eye, by infra-red light.
Such phenomena as these have a direct bearing upon road safety and upon many branches of industry. Motor car headlights are tested in a gallery which extends for the entire length of the building — 145 feet. Searchlights are tested in the open air, and lamps developed for road lighting may be mounted at any desired height on a standard, measurements of illumination being taken on the “road surface” on the floor beneath.
Photometry, in its strict sense — the measurement of the relative intensity of light — is carried out, in general, by photoelectric methods. A new technique has been evolved in recent years; instead of measuring the candle power of a lamp in one particular direction, the total amount of illumination given by the lamp is measured by placing it inside a sphere, in which is a photoelectric cell screened from the direct radiation from the source of light.
Life tests of commercial electric lamps are made, their candle power being checked at specified intervals. Rear reflectors for bicycles are tested to determine whether, they satisfy the requirements laid down by the Ministry of Transport.
Visibility in lighted streets is studied m ingenious ways. One method adopted is to throw on a screen a cinematograph picture of a lighted street in which objects appear from time to time at selected points. These objects have to be detected and reported upon by an observer who is unfamiliar with the film or with the test being carried out. By using different types of lamp in the projector, the road illumination that would be given by these various lamps is simulated in the projected picture.
Through holes in the screen, brilliant points of light may be made to give the effect of glare. The readiness with which objects in the street are picked out by observers in various conditions is noted.
Standardizing Signal Lamps
For photometric tests of high-power lamps — and particularly of electric discharge lamps — a huge integrating sphere photometer, 10 feet in diameter, is used. Directional tests with large lamps and with lanterns designed for use with them are made by plotting polar diagrams, using the artificial “road” previously mentioned.
Other investigations carried out relate to the design of ships’ navigation lights, railway and road signal lights, the standardization of the coloured glasses used in such lights, and to many similar problems.
An interesting feature of the Photometry Division of the N.P.L. is the distribution board from which the electricity supply to various branches of the building is controlled. In complexity it resembles an automatic telephone exchange switchboard, but all loose leads have been eliminated, and control is flexible and simple.
The wide diversity of the work carried out in the Electricity Department is a striking testimony to the indispensability of electricity in the service of the engineer. The electrical engineer is a specialist, but his fellow workers, whether their task be bridge building, road making, or general engineering practice, are all dependent upon electricity for some purpose or in some form.
The Electricity Department of the National Physical Laboratory and the work undertaken by its staff of scientists are of vital importance to the engineer and to the entire community.
HUGE TRANSFORMER for the generation of high voltages. The rapid development of the transmission of electrical power has brought in its train many problems which can be solved only with the help of a laboratory fully equipped for high-voltage work. Such work as the testing of the insulators used on the national transmission lines is carried out in this laboratory.