PART TWO: POWER AND ENGINEERING 382 The first company to exploit electric motors on a large scale was Siemens, and their first major customer was the Prussian state mines. Electricity in mining, however, developed fairly slowly. Although some mine winders were electric before the First World War it was only in the 1920s that mine electrification became widespread. A major industrial use of electric power was the iron and steel industry. Several ironworks adopted electric lighting quickly because it facilitated all-night working, so perhaps ironmasters were easily alerted to the possibilities of the new power. Electric motors proved to be very good for driving rolling mills, where the combination of power and precise control was valuable. Thereafter electric motors were gradually introduced for driving machine tools, and the advantages of individual drives over line-shaft systems were readily appreciated. The first permanent public electric railway was opened at Lichterfelde in Germany in 1881. Built by Siemens, it ran for about three kilometres. Each carriage had a motor under the floor connected to the wheels through a belt drive. The first electric railway in the United Kingdom ran between Portrush and Bushmills, in Ireland. Electric traction was chosen there because abundant water power was available and a hydro-electric generating station was built on the River Bush. England’s first electric railway was the Volk’s Railway which still runs on the sea front at Brighton. The railways just mentioned were all small systems. The first really practical electric tramway system was built by Sprague in Richmond, Virginia, in 1888. Forty cars powered from overhead conductors ran over 20km (12.5 miles) of streets. Frank J.Sprague trained as an engineer with the US navy, then set up his own electrical engineering company in 1884. His most important contribution was the multiple unit control system, which made it possible to have motors distributed along the length of a train and supplied from current collectors on each coach but all controlled from the driver’s cab. He also introduced the motorized bogie construction in which one end of the motor is pivoted on an axle and the other supported on springs. Electric trams were introduced only slowly in Britain, partly because the established horse-drawn tramways were approaching the date when, under the Tramways Act, they could be purchased compulsorily by the local authorities. New tramways after 1890 were virtually all electric. Deep tube railways in London and other cities only became practicable with electric traction. The first was the City and South London Railway which ran initially from Stockwell to the Bank. The original rolling stock was fourteen locomotives each of which could haul three carriages with thirty-four passengers in each. Five million passengers were carried in the first year. Electricity was generated in a specially built power station at Stockwell, and supplied through a third rail. The service interval at periods was just under four minutes. Most early electric traction systems used direct current because the DC series wound motor has good operating characteristics for the purpose. In ELECTRICITY 383 1920 the Ministry of Transport adopted 1500 volts DC as the standard for main line railways. Technical advance soon forced a change in the standard, however. The introduction of the mercury arc rectifier in 1928 made it possible to transmit AC and convert to DC on the train; most of British Rail now uses this system, with 25kV overhead lines. Since about 1960 semiconductor rectifiers have been available for high powers, and are replacing the mercury arc rectifiers. The Southern Region, where the traffic density is higher than in most of the country, uses a third rail for the conductor and operates at about 700 volts DC. Other countries have used a variety of systems and frequencies. Switzerland and Italy have some three-phase railways, with one phase earthed to the running rails and the other two phases connected through two overhead wire systems. The advantages of alternating current transmission encouraged engineers to develop AC motors. Most DC motors will in fact operate on AC supplies, provided that the iron cores in their fields are laminated, but they are not so efficient. Such machines are called universal motors, and are often found in small domestic appliances such as vacuum cleaners and food mixers. The first practical AC motors were developed by Nicola Tesla in 1888. He was born in Austria-Hungary but in 1884 emigrated to the USA where he spent most of his life. He worked for a time for Edison, the leading exponent of DC systems in the USA, then joined Westinghouse, the leading AC man. In his machines Tesla made use of the fact, discovered by Arago in 1824, that a piece of magnetic material free to turn will follow a rotating magnetic field. He created a rotating magnetic field by using two coils energized from supplies that were in synchronism but not in phase. His first machine had two coils with axes at 90° to each other supplied with alternating currents also 90° out of phase. He showed that the resultant of the two oscillating magnetic fields was a rotating field, and he performed the same analysis for a three- phase system at 120°. Tesla’s first motor was a synchronous one—that it, the rotating member either was or became a permanent magnet, and its poles followed the rotating field round, keeping in synchronism. He also made induction motors, in which the rotating member is not a permanent magnet and turns at a speed slightly lower than the speed of the rotating field. Currents are then induced in the rotor and these interact with the rotating field to provide the driving force. Other people were also working on the idea of a motor driven by a rotating magnetic field, and Tesla’s patent claims were challenged in the US courts, but his claims were upheld. He also showed that an induction or synchronous motor can be run from a single phase supply if part of the field winding is connected through a capacitor or inductor to give a second phase. Once started, such a motor will run satisfactorily on a single phase supply. The PART TWO: POWER AND ENGINEERING 384 Westinghouse company bought Tesla’s patents, and from 1892 they were manufacturing AC motors and promoting AC supply systems. In the twentieth century most of the world’s electric motor power comes from induction motors. Their disadvantage is that they are essentially constant speed machines, but for many applications that is perfectly satisfactory and the inherent simplicity and robustness of induction motors, which usually have no brush gear, make them the first choice for many applications. Most domestic washing machines are driven by an induction motor with capacitor start. A motor that is sometimes confused with the induction motor is the repulsion motor, developed by Elihu Thomson and Professor J.A.Fleming. Fleming, better known for his work on radio, studied the forces between conductors carrying alternating currents. In 1884 he showed that a coil carrying alternating current tries to position itself edge on to a magnetic field, and the force produced in this way provides the basis for the repulsion motor. A typical repulsion motor has a single field coil connected to the supply and a wound multi-coil armature with commutator. Two brushes on opposite sides of the commutator are connected together (but not to the supply) and short circuit the armature coils which at that instant are across the magnetic field. There is then a turning force which moves the armature. Repulsion motors have been used for electric traction. They have a good starting torque and some speed control is possible by moving the brushes. MODERN ELECTRIC MOTORS By the mid-twentieth century it seemed reasonable to say that electric motor development was complete, but in 1957 Professor G.H.Rawcliffe developed the pole amplitude modulated, or PAM, motor. This is a synchronous or induction motor whose field coils are so arranged that by interchanging a few connections the number of poles can be changed. Since the speed is determined by the number of poles (as well as by the supply frequency) this gave a motor whose speed could be switched between two distinct values. A PAM induction motor therefore retains the reliability and robustness of the conventional induction motor but can work at two different speeds. The other approach to variable speed control is to change the supply frequency. With power semiconductors that is becoming possible. By 1960 semi-conductor devices were available capable of controlling a few tens of amperes, but progress in the following decade was so rapid that by the end of it semi-conductor frequency convertors were available capable of supplying the largest motors—and controlling their speed. Another area of motor research that remains active is linear motors. Often described as conventional motors that have been slit open and unrolled, linear motors have been a subject of research at least since 1841, when Wheatstone ELECTRICITY 385 made one. The idea was revived in 1901 when the Norwegian Kristian Birkeland tried to use a linear motor as a silent gun. Their best-known modern exponent is Professor Eric Laithwaite, who was first interested in them as a means of driving a weaving shuttle. Linear motor research continues, and the widespread application of these machines will probably use semiconductor controls also. THE STEAM TURBINE In nearly all the early power stations the prime movers were reciprocating steam engines. The technology was well established, and the electrical designers made generators to be driven by the available steam engines even though their rotational speed was less than ideal. Some stations used a belt drive to increase the speed, though Willans and Belliss & Morcom high speed engines were often directly coupled to the generator. The higher rotational speed of the turbine made it the ideal prime mover for power stations. Generators for use with turbines have usually only a single pair of field poles, rather than the multi-polar machines used with reciprocating drives. Initially the armature windings, in which the current was generated, were the rotating member and the field poles were static. As machines became larger, it became difficult to make brushes and slip rings or commutators adequate to take the current. The solution was to ‘invert’ the machine, having the armature static and the field rotating. The brushes then had to carry only the magnetizing current for the field. The last large rotating armature machine was a 1500kW set for Neptune Bank power station on Tyneside in 1901. The first rotating field generators had salient poles built up on the rotor shaft. The Anglo-Swiss engineer Charles Brown, of the Brown-Boveri partnership, proposed that the rotor should be a single forging and that the windings should be carried in slots milled in the surface. This basic design has been used for large generators ever since. The largest turbines and generators now used by the Central Electricity Generating Board, serving England and Wales, are rated at 660MW. Today the entire electricity demand of the United Kingdom could be supplied from only one hundred generators. ELECTRICITY TODAY Modern life depends on electricity. Virtually every home in Britain is connected to the public electricity supply, though that has been achieved only since the Second World War. In 1920 the supply industry had under a million customers in England and Wales. The figure reached 10 million by 1945 and PART TWO: POWER AND ENGINEERING 386 15 million by 1960. Now there are 21 million, and they use about two hundred thousand million units of electricity per year. Of this consumption 36 per cent is used at home, 38 per cent in industry, 22 per cent in commerce, and the remaining 4 per cent in such diverse applications as farming, transport and street lighting. Of the domestic electricity, 21 per cent is used for space heating, 18 per cent for water heating, 11 per cent for cooking and 17 per cent for freezing and refrigeration. Everything else, including lighting, comes out of the remaining 33 per cent. At first electricity was only for the well-to-do. The major expansion came during the 1920s and 1930s, and during that period the average consumption per household fell, reflecting the fact that new consumers used electricity mainly for lighting, and not much for other purposes. The range of domestic electrical appliances with which we are familiar today have in fact been available almost from the beginning. Catalogues of the 1890s include electric cookers, kettles, saucepans, irons and fires. Early electric fires used carbon filament lamps as the heating member because there was no metal (except platinum) which could be heated to red heat in air without oxidizing. A great advance came in 1906 with the alloy nichrome, a mixture of nickel and chromium. This does not oxidize when red hot, and most electric fires since that date have used nichrome wire elements on fireclay supports. Storage heaters for room heating were introduced on a small scale in the 1930s. In the 1960s the Electricity Council conducted research to improve their design, seeking longer heat retention, and modern storage heaters are much smaller than their earlier counterparts. Motorized appliances generally came later than lighting and heating, though an electric table fan was on sale by 1891. The first electric vacuum cleaner was made in 1904. Early electric washing machines had a motor fixed underneath the tub. Usually there was a mangle fitted on top (spin driers came later) and a gearbox that permitted the user to couple the motor either to the agitator in the tub or to the mangle. Food mixers and refrigerators came after the First World War, though they were rare until the 1950s (see Chapter 19). Electric space heating and refrigerators have changed house design. Before the mid-1930s it was normal to have a fireplace in every bedroom, and into the 1950s every house was built with a larder. Many modern houses have no fire- place, except possibly one in the living-room for effect. Larders have become obsolete since it is assumed that food which might go bad will be kept in the refrigerator. Lighting has also progressed. The carbon filament lamps that were such a wonder in the 1880s and 1890s encouraged the gas industry to develop the mantle, and for a time gas lighting undoubtedly had the edge over electricity. The electric lighting industry sought a better filament material. Three metals seemed promising: osmium, tantalum and tungsten. Osmium filament lamps were on sale from 1899, but since 1909 all metal filament lamps have used ELECTRICITY 387 tungsten. Carbon lamps continued to be made for some years since they were cheaper in first cost, but the metal filament lamps were more efficient, giving cheaper light when the cost of electricity was taken into account. The latest development in high-power filament lamps is the inclusion of a halogen gas (usually bromine or iodine). This reacts chemically with tungsten that evaporates from the filament and is deposited on the glass. The resulting tungsten halide is a gas which decomposes close to the hot filament, depositing the tungsten back on to the filament. Such lamps can be run at a higher temperature and are therefore more efficient. Various gas discharge lamps were made in the 1890s, and neon lamps were introduced about 1910. The widespread use of both mercury and sodium discharge lamps dates from the 1930s. The low pressure sodium lamp, with its extremely monochromatic yellow light, has been popular for street lighting because it is the most efficient of all. Since the early 1970s, however, the high pressure sodium lamp has been taking over. It is almost as efficient, and although its light has a yellow-pink tinge its colour rendering ability is fairly good. Fluorescent lamps, developed in Britain just before the Second World War, have an efficiency in between that of filament lamps and discharge lamps. A low pressure mercury discharge within them produces ultra-violet light which acts on the fluorescent coating of the tube to give visible light. The choice of phosphor determines the colour and the efficiency of the lamp, and they are widely used in commercial applications. One great advantage of electricity is its easy controllability, and with time- switches, thermostats and semi-conductor dimmers that is even more true than before. Other technologies have done much for mankind: electricity has put virtually unlimited power at the disposal of all. 388 7 ENGINEERING, METHODS OF MANUFACTURE AND PRODUCTION A.K.CORRY INTRODUCTION ‘Man is a tool using animal’ said Thomas Carlyle but, additionally and more significantly, man has progressively designed and made tools for use in meeting his developing needs and adapted his techniques and social organization to make the best use of his skills of mind and body in manipulating materials to his advantage. Side by side with the development of hand tools the principles of work organization were being realized in tribal cultures by making the best use of the special skills of individuals in making and maintaining tools for the experts using them: the relationship between hunter and spearmaker is an example of this division of labour. Progressively it was also recognized that there exist three basic elements in tool design. The first of these, the need for a cutting edge harder than the material to be worked, is the most fundamental and delayed wider development until the discovery of metals. The other two factors (which are also dependent to an extent on cutting tool materials) are the need to minimize demands on human energy and the search for substitutes for manual skill; all three are the subject of research and development to this day. BRONZE AND IRON AGE TOOLS It is probable that bronze was the first metal used as a tool material although iron was known about at an early date. There are many references to iron in the Bible and its superiority to bronze. Goliath is described as wearing bronze ENGINEERING AND PRODUCTION 389 armour and having a spear pointed with iron, and there are also mentions of steel. Aeschylus, c. 480 BC, writes about the superiority of iron for weapons. The greater difficulty of extracting and forging iron, and the effect of corrosion, extended the use of bronze in weapons, armour and tools until the techniques of working iron were developed to demonstrate its superior qualities in providing and keeping a keen cutting edge. By the end of the Roman Empire virtually all the forms of modern hand tools had been devised; the second major step in the development of manufacturing was already under way by the introduction of mechanical means of enhancing the force of cutting and forming to take advantage of the high cutting speed possible with iron tools. The first mechanically assisted cutting operation was drilling and the earliest example is the rocking drill, the most important method being the cord drive whereby an assistant manipulates a cord wrapped round the vertical drill spindle to give it an alternating rotary movement. This system, applied horizontally, was probably used to drive the lathe spindle which produced the first extant example of turning: an Etruscan wooden bowl found in the Tomb of the Warrior at Corneto, c. 700 BC. The earliest illustration of this type of lathe, on a wall of the Egyptian tomb of Petosiris (third century BC), shows an assistant holding each end of the cord to give the rotational movement to the spindle. The Egyptian figurative convention confusingly shows the spindle vertical, but it illustrates the provision of bearings and tool rest for accurate positioning of the cut being made and to take the load imposed on the workpiece by the cutting action. These requirements, for load bearing and tool rigidity in relation to the workpiece, have continued to be principal elements in machine tool design, together with work holding and spindle drives. The Kimmeridge ‘pennies’ discovered at the Glastonbury Lake Village, Somerset, were turned from soft stone c. 100 BC and show interesting methods of attaching and driving the workpiece from the spindle. One has a roughly drilled hole used to mount the work on a shaft or mandrel which in turn is held between centres similar to the Egyptian lathe. Others show the use of a squared hole to permit driving from a similarly squared spindle nose and the small centre holes necessary for head and tail centring. Spindle driving methods advanced slowly. Although the ability to turn in stone led to the spindle-mounted grindstone with a turned true outer surface, for maintaining cutting edges on tools and weapons, the cranked arm used for driving the wheel in the earliest illustration of it on the Utrecht Psalter, AD 850, was not used for a lathe spindle until the second half of the fifteenth century. Similarly the bow replacement for the cord drive operated by an assistant was not used for lathe drive until the Roman Empire. This method is still used in the Middle East by wood turners and watchmakers in the Western world were using bow drills in the early part of this century. The difficulty of manipulating the bow while guiding the tool with intermittent cutting, calls for a very high degree of manual skill and dexterity and it is only possible to make light cuts. PART TWO: POWER AND ENGINEERING 390 EARLY MACHINES As the size and complexity of work for the lathe increased so did the need for increased rigidity and more power, which was met by heavier construction of the wooden frames of lathes and the development of the pole lathe. This still gave intermittent cutting, but freed the turner’s hand to concentrate on guiding the tool by the use of a spring pole to which one end of the cord was attached and the other end to a foot operated treadle after passing round the work to be turned. When the treadle was pressed the work revolved towards the turner for cutting, and on completion of the treadle throw the spring pole returns the work. This type of drive probably existed in the twelfth century; the best early illustration of the pole lathe occurs in the Bible Moralisée, c. 1250 (see Figure 7.1). The use of this type of lathe has continued in similar form until well into the twentieth century with the ‘chair bodgers’ at work in the woods around High Wycombe in Buckinghamshire. The pole lathe, with its intermittent cut, was not adequate for turning metal and, as the need for machined metal products increased, the continuous method of driving was developed, first of all through the use of a large wheel in separate bearings carrying round its periphery a cord which also passed round the work spindle. The large wheel was turned by an assistant using a cranked arm, first illustrated c. 1475 and also in Jost Amman’s Panoplia of c. 1568. The next method of continuous driving using a treadle and crankshaft, shown by Leonardo da Vinci, c. 1500, in the Codice Atlantico and developed by Spaichel, c. 1561, still gives a satisfactory system for the ornamental turners, sewing machines and other machines where only human power is available; Figure 7.1: Miniature painting of a pole lathe, c. 1250, from the Bible Moralisée. ENGINEERING AND PRODUCTION 391 but it was the large wheel and continuous band method which was to enable the use of other power sources: horse gins, water wheel, steam engine and electric motor. The development of continuous drive also made possible the control of the cutting tool relative to the work through systems of gears, screws and guides progressively to eliminate the skill required in holding and guiding the cutting tool and make use of the ideas first expressed according to drawings in the Mittelalterliche Hausbuch, c. 1480, of tool holder and cross slide with screw cutting lathe. MEASUREMENT From the earliest days of man’s use of tools, measurement of the size and shape of things produced has been of prime importance to satisfy the performance required. The earliest standards were those designed to meet individual needs, but these were gradually developed to use units of measurement which could be employed to reproduce articles in a range of sizes. The first ‘standard’ was the Egyptian Royal Cubit, equivalent to the Pharaoh’s forearm length plus palm and made of black granite. This master standard was subdivided into finger widths, palm, hand, large and small spans, one remen (20 finger widths) and one small cubit, which was equivalent to six palms. The small cubit was used for general purposes and made in granite or wood for working standards. These were regularly checked against the master, and many Egyptian temples and other buildings had reference measures cut into walls to check the wooden cubit which, being much easier to handle than stone although more prone to variation, came into more general use. These principles and the use of cubits and their subdivisions became the basis of Roman, Greek and Middle Eastern measures and later European measures, although the actual ‘reference factor’, the forearm, produced some alternative cubits in different parts of the world. All these standards were ‘line standards’ involving measuring between engraved lines and this remained the basis of national and international standards until 1960, when the concept of keeping a physical standard was abandoned in favour of the wavelength of krypton 86 which is a readily reproducible and constant reference factor. Once standards of length were established, these were used to check parts for accuracy and measuring tools, for use in transferring sizes for comparison with the standard, were developed. Calipers, dividers and proportional dividers were evolved by Greeks and Romans some 3000 years ago. These instruments, with the cubit measuring stick, made possible accurate calculations, by measuring the shadow cast by the sun, to determine heights of buildings, agree the time of day, establish the calendar and navigate by reference to the stars. From this point the development of metrology has been . coating of the tube to give visible light. The choice of phosphor determines the colour and the efficiency of the lamp, and they are widely used in commercial applications. One great advantage of. of the Roman Empire virtually all the forms of modern hand tools had been devised; the second major step in the development of manufacturing was already under way by the introduction of mechanical. means of enhancing the force of cutting and forming to take advantage of the high cutting speed possible with iron tools. The first mechanically assisted cutting operation was drilling and the earliest