An Encyclopedia of the History of Technology part 39 docx

PART TWO: POWER AND ENGINEERING 362 ‘by eye’. The detailed design of the field magnets and coils was studied by John Hopkinson, Professor of Electrical Engineering at King’s College London, and a consultant to the American electrical inventor Thomas Alva Edison. Seeking to introduce a complete electric lighting system, with lamps, generators and other equipment all of his own design, Edison had been making generators with very long field poles and coils. Hopkinson made a number of small models of field systems of different shapes, and measured the magnetic field produced at the armature. As a result he concluded that Edison’s poles were much too long, and he designed a fairly squat machine whose general proportions were followed by many manufacturers. The machines described above were for direct current. A different pattern was adopted for the early alternating current generators. These normally had the armature coils arranged around the edge of a fairly thin disc and moving between the poles of a multi-polar field system. This design gave a machine whose reactance was low—important on an alternating current system—and allowed a sufficient number of poles to be used. It was essential to use multipolar machines if the generator were to be coupled directly to a steam engine. Even the fastest reciprocating engines ran at only about 500rpm. A twelve-pole generator running at that speed would give a 50Hz output. In practice supply frequencies varied from 16.6 to 100Hz. The disadvantage of the disc generator was that it was impossible to make such a machine for three- phase operation. However, before three-phase supplies came into general use the turbine had replaced the reciprocating steam engine, and generators were being designed for the higher running speed of the turbine. ARC LIGHTING Although the possibility of electric arc lighting had been demonstrated very early in the nineteenth century, it could not be a practical proposition until a supply of electricity was readily available. The development of satisfactory generators in the 1870s stimulated fresh interest in the possibility of electric lighting. In an electric arc lamp two carbon rods are connected to the opposite poles of the supply. The rods are briefly touched together and then drawn a few millimetres apart. This draws a spark, or ‘arc’, which continues as long as the electricity supply is maintained. The current in the arc produces considerable heat, and the contact points on the carbons quickly become white hot. These white hot places on the carbons are the source of the light. White hot carbon burns in air, and so some arrangement is necessary to feed the carbons closer together so that the gap is kept constant. Without any adjustment the gap widens and within a minute or two the electricity supply will be unable to maintain the arc across the wider gap and the lamp will be extinguished. ELECTRICITY 363 The first arc lamps were manually adjusted, and for applications such as theatre spotlights there was no problem about having a man constantly in attendance. For general lighting, however, the arc would only be acceptable if some reliable method could be found for adjusting the carbons automatically. Much inventive ingenuity went into devising suitable mechanisms for the purpose. The first arc lamps to be used in quantity for general lighting were known as Jablochkoff candles. Paul Jablochkoff was a Russian telegraph engineer who set out from Russia in 1875 intending to visit the United States centennial exhibition at Philadelphia in 1876. He only got as far as Paris, where he became interested in electric lighting, and it was in Paris that he invented his ‘candle’. Jablochkoff s candle consisted of two parallel carbon rods placed side by side but separated by a thin layer of plaster of Paris. At one end the rods terminated in brass tubes which secured the candle in a holder and made the electrical connections, at the other end they were joined by a thin piece of graphite. When the candle was in its holder and the current was switched on, the graphite fused, starting an arc between the ends of the carbons. As the carbons burned the plaster crumbled away in the heat, exposing fresh carbon. Provided the candle had been well made the carbon and the plaster were consumed at the same rate and the result was a steady light. However, once the light was extinguished, for whatever reason, it could not be restarted. For street lighting this did not necessarily matter: the candle would last for an evening, and during the next day a man could go round putting in fresh candles. Automatic mechanisms were made which brought a new candle into the circuit when the first was extinguished, but the candle itself soon became obsolete as regulating mechanisms were devised which could be mass produced. The Jablochkoff candle was only used for a few years, but it was used prominently. It was first installed in Paris, attracting much attention. In July 1878 the London technical journal The Electrician complained that ‘The application of the electric light is in Paris daily extending, yet in London there is not one such light to be seen.’ In October the same year the Metropolitan Board of Works arranged a trial of electric lighting, using Jablochkoff candles, on the Victoria Embankment. About the same time the City of London authorities arranged some trials in front of the Mansion House, on Holborn Viaduct and in Billingsgate fish market. All these installations were working by Christmas 1878. Arc lamp regulators have to perform two distinct functions. First the carbon rods must be brought together and then drawn apart when the current is turned on, and secondly the spacing of the rods must be maintained. The first function was quite easy to achieve: the upper carbon was allowed to fall under gravity and make contact with the lower. An electromagnet connected in series with the lamp then pulled the lower carbon down a few millimetres to start the PART TWO: POWER AND ENGINEERING 364 arc. The rate of fall of the upper carbon was controlled by a brake, and the second function of the mechanism was to control the brake. The smooth operation of the lamp depended on the brake, and any sudden movement of the upper carbon would cause the light to flicker. Most of the earlier lamps used the series electromagnet to control the brake. This was easily arranged: as the gap widened and the arc lengthened the current would fall, and the electromagnet, which had been holding the brake on, would weaken, releasing the brake until the gap was restored (see Figure 6.7). The disadvantage of using the series electromagnet to control the arc lamp was that only one lamp could be connected to the supply. If two lamps were connected in series to one generator, then a fall in current due to one gap widening would affect both regulators, and the brake on one lamp would be released too soon. (Arc lamps will not operate in parallel, either, because the arc is a negative resistance and one lamp would take all the current while the other went out.) More satisfactory arc lamps were made by introducing another electromagnet, connected in parallel with the arc. As the arc widens the current falls, but the voltage across it increases. The parallel electromagnet therefore became stronger as the gap widened, and it was used to release the brake which was normally held on by a spring. Most arc lamps from the mid-1880s onwards worked in this way. Many designs were made, with the object of producing a cheap, reliable, yet sensitive control mechanism. Arc lighting was widely adopted in places where it was suitable—in large buildings like markets and railway stations and for street lighting. King’s Cross Figure 6.7: Compton arc lamp mechanism of about 1878. ELECTRICITY 365 Station in London, for example, was lit by twelve lamps in 1882. The lamps, rated at 4000 candle power, were hung ten metres above the platforms and supplied from four Crompton-Burgin generators all driven by a single steam engine. Two improvements in arc lighting were made during the 1890s. One was the enclosed arc, which had the arc contained within a small glass tube that restricted the air flow. The effect of this was to reduce the rate of burning of the carbons. The second improvement was the addition of cores of flame- producing salts, mainly fluorides of magnesium, calcium, barium and strontium, to the carbon rods. They increased the light output, and also gave some control over the colour of the light. In 1890 there were reported to be 700 arc lamps in use for street lighting in Britain, and probably a similar number in private use. About 20,000 were installed in the following twenty years, but by then the filament lamp had been developed to an efficiency at least equal to that of the arc. Although few, if any, further arc lamps were installed, those that were already in place continued in use. London retained some arc street lighting into the 1950s. THE FILAMENT LAMP There was no call for a public electricity supply until the invention of a satisfactory filament lamp. Electric arc lighting was proving its worth for streets and public buildings, but it was quite unsuited to domestic use. The individual lamps were far too bright, too complex, and too large physically for use in the home, and they would probably also have been a fire hazard. The idea of the filament lamp was almost as old as the arc lamp. Many people had tried to produce light by heating a fine wire electrically so that it glowed, but they all faced a series of seemingly insuperable problems. They had to find a material that would stand being heated repeatedly to white heat and then cooled, then it had to be sealed into a glass vessel in such a way that the glass did not crack when the wire was hot, and finally the air had to be pumped out so that the filament did not oxidize. The early attempts at making a practical incandescent filament lamp all failed, mainly because of the difficulty of obtaining an adequate vacuum. After the invention of the Sprengel mercury pump in the mid-1870s several inventors succeeded in making viable lamps, the best known being Edison and Swan. At the first International Electrical Exhibition, held in Paris in 1881, four manufacturers had filament lamps on display: Swan and Lane-Fox from Britain and Edison and Maxim from the USA. There was little to choose between the four lamps at the exhibition, though Swan and Edison soon captured the market while the others disappeared from the scene. Swan and Edison were very different men, with different approaches to their common objective of developing an electric light suitable for domestic PART TWO: POWER AND ENGINEERING 366 use. As Sir James Swinburne later remarked, ‘Edison and Swan were hardly racing, as they were on different roads.’ Joseph Wilson Swan, later Sir Joseph, was born in Sunderland and apprenticed to a firm of druggists there. Subsequently he set up in business with his friend John Mawson as chemists and druggists in Newcastle upon Tyne. A man of wide scientific interests, he was intrigued by some of the early experiments towards an incandescent filament lamp, and made several experimental lamps himself in the 1840s and 1850s. He made carbon filaments from strips of paper treated with sulphuric acid to give a very smooth material called ‘parchmentized paper’ because of its resemblance to parchment. He carbonized the paper and mounted it in a glass vessel closed with a rubber stopper and then evacuated. However, he could not obtain a sufficient vacuum, and left the experiments for twenty years during which time he worked on other matters, especially photography: the most important of his inventions in that field was the silver bromide photographic paper still used for black-and-white prints. In 1867, John Mawson, who was by then his brother-in- law as well as close friend and colleague, was killed in an accident. Swan found himself responsible for the families and for a large chemical business. About 1877, Swan was able to resume his interest in electric lighting, and the new Sprengel airpump gave him fresh impetus. He first spoke publicly about it on 19 December 1878, at an informal meeting of the Newcastle Chemical Society when several members gave brief talks. It seems probable that he did not have a working lamp to demonstrate then, but he certainly did so at several public meetings in the area in January to March 1879. During 1879 he worked to improve his lamp. The main problem, which was discussed at another Newcastle Chemical Society meeting on 18 December 1879, was that residual gas was occluded in the carbon and came out when the filament became hot. This gas carried particles of carbon which were deposited on the cooler glass, causing blackening. The solution was to pump the lamps while the filament was hot, and Swan applied for a patent for that process on 2 January 1880. He never tried to patent the basic idea of a carbon filament lamp, since he considered that there was nothing novel in that. During 1880 he worked on the filament material. While studying the problems of evacuating and sealing the lamp he had used mainly thin arc-lamp carbons for his filament—they were available only about one millimetre in diameter, and were relatively strong. The carbonized paper and carbonized parchmentized paper were not entirely satisfactory. He tried other substances and found a suitable material in parchmentized cotton, which was ordinary cotton thread treated with sulphuric acid, as the parchmentized paper had been. This gave a compact and uniform material which could be carbonized in a furnace to give satisfactory filaments. Swan applied for a patent for this filament on 27 November 1880, and went into commercial production in 1881 (see Figure 6.8). He never sought to manufacture other components for an electric lighting system, although he worked closely with Crompton who was making generating equipment and arranging electrical installations. ELECTRICITY 367 Edison’s approach was quite different. He had already made a name for himself as an electrical inventor, and had built up a research organization. He became interested in electric lighting late in 1877, after visiting William Wallace’s electrical factory in Connecticut. Wallace made arc lighting equipment. Edison thought that a viable electric lighting system should have lamps of about the same power as the gas jets then in use and that electricity should be distributed in a similar way to gas, with each light being independently controlled. He wanted to produce both the lamps and the electricity supply system to feed them, and all the resources of his laboratory and staff were turned to the subject. Edison’s friend, the lawyer Grosvenor P.Lowrey, put up $300,000 to establish the Edison Electric Light Company in October 1878. It was in Figure 6.8: Advertising drawing published by the Ediswan company in the late 1930s, showing Swan’s first successful filament lamp of 1881. PART TWO: POWER AND ENGINEERING 368 that month that Edison announced publicly: ‘I have just solved the problem of the subdivision of the electric light.’ The search for a viable filament lamp was often called the problem of ‘subdividing’ the electric light because the perceived need was for a much smaller lighting unit than the arc light. Edison’s announcement, which received wide publicity and caused an immediate slump in gas shares, was based on a lamp with a platinum filament. This lamp contained a thermostat which momentarily cut off the current when the filament was in danger of overheating and melting. It was not until late 1879 that Edison turned again to carbon as his filament material. When he began commercial production of filament lamps in 1880 he used filaments made from Bristol-board—a thick paper with a very uniform texture. Continuing the search for better materials, however, he found that fibres from a particular variety of bamboo gave the best results, and used that from mid-1881 to about 1894. Several other people made workable filament lamps, and at least two of them went into commercial production. Hiram S.Maxim, who is better known now for his work on guns and on aerial navigation, was an American by birth, but later became a naturalized Briton and was knighted. The other was an Englishman, St George Lane-Fox, who also designed a complete distribution system. His patents were acquired by the Anglo-American Brush Electric Light Corporation. The manufacture of Swan’s first commercial lamps was a complex enterprise. The ladies of the Swan household in Newcastle upon Tyne prepared the filaments and Swan himself carbonized them. The bulbs were blown by Fred Topham and all the components were conveyed to Birkenhead where C.H. Stearn mounted the filament assemblies in the bulbs and evacuated them. A catalogue published by the Swan United Electric Light Company in 1883 lists more than a hundred houses and other buildings and twenty-five ships lit with Swan’s lamps. Probably the most prestigious contract was for lighting the new Law Courts in London, which opened in December 1882. Crompton supplied the generators and six arc lamps for the large hall, and Swan supplied filament lamps for the courts and other rooms. A number of large private houses were lit electrically, using current supplied from their own generating plant. Sir William Thomson lit his house in Glasgow. In a letter to Sir William Preece, who was lighting his house in Wimbledon, Thomson noted the need for lampshades. ‘The high incandescence required for good economy is too dazzling and I believe would be injurious to the eyes if unmitigated. I have found that very fine silk paper round the globe spreads out the light quite sufficiently to make it perfectly comfortable to the eye while consuming but a small percentage of the light and Lady Thomson has accordingly ELECTRICITY 369 made little silk-paper globes for nearly all our lights.’ He goes on to say that there were 112 lights in the house. CENTRAL POWER STATIONS The first central electricity generating station offering a supply of electricity to the general public was probably the one that began operating at Godalming, Surrey, in the autumn of 1881. Until that year the streets of Godalming were lit by gas, under a contract that expired at the end of September. In 1881 the town council and the gas company were unable to agree on the price to be charged for the coming winter’s street lighting. An influential figure in the town was John Pullman, of R. & J.Pullman, leather dressers, who had a business based at Westbrook Mill, on the River Wey. It was probably Pullman who suggested that the town should have the new electric light rather than gas; he offered the use of his waterwheel to drive a generator in exchange for free light at the mill. The apparatus was soon installed. A Siemens generator at the mill supplied seven arc lamps and about forty Swan filament lamps. A few of each type of lamp were at the mill, the remaining arc lights were in the main streets of the town and the filament lamps in the side streets. It was announced that people who wanted electric lighting could have the wires taken into their homes. Although the lighting created great local interest and was reported in the local and national press, very few people took up the opportunity. In May 1882, Sir William Siemens said there were only ‘eight or ten’ private customers with a total of 57 lamps between them. The installation was never a commercial success, though the Siemens company felt that they learned useful practical lessons from it before, in 1884, the town reverted to gas lighting. The River Wey proved to be an insufficiently reliable source of power for electric lighting, and within a few months the generator was removed from Pullman’s mill and set up in the town centre, where it was driven by a steam engine. That move also helped with the solution to another problem, volts- drop in the wires. It was soon found that the voltage at the end of the supply cables was less than the voltage at the generator, and that therefore if the voltage was right close to the generator, then lamps at the other end of the circuit only glowed dimly. This problem was reduced when the generator was moved to the town centre, but it lead Sir William Siemens, in evidence to a House of Commons Select Committee, to express the opinion that no electricity supply station could be more than about half a mile from its most distant customer. The Select Committee was considering the Bill which became, in August 1882, the world’s first Electric Lighting Act. This Act laid down a general PART TWO: POWER AND ENGINEERING 370 legislative framework for electricity supply, and was drafted on the assumption that an electricity supply undertaking would be a fairly local matter, and that the local authority should, if it so wished, have a large measure of overall control. At least three other public supply systems were in operation before the Act came into force. Edison had an experimental steam-powered station in London, at Holborn Viaduct, which began supply in January 1882 and was really a trial for his first New York station, which opened in September of the same year. Crompton extended a street lighting and market lighting installation in Norwich to supply private houses from about March 1882. Perhaps the most important of these early schemes, however, was the one at Brighton, Sussex. The electrical pioneer Robert Hammond had visited Brighton in December 1881 to give an exhibition of Brush arc lighting. The demonstration was so successful that Hammond was asked to extend it, and on 27 February 1882 he opened a permanent supply undertaking. Brighton has had an electricity supply for longer than anywhere else, for all the other very early undertakings closed within a short period. A supply undertaking that did not use overhead wires and did not need to break up the streets could operate outside the provisions of the Electric Lighting Act, 1882. The largest company to use that loophole was the Kensington Court Electric Light Company. Kensington Court was a new housing development just south of Kensington High Street, in west London. About a hundred houses on the estate were linked by a system of subways, in which the company laid its mains. Crompton was the leading figure in the Kensington Court Company, which was registered in June 1886 and commenced supply in January 1887. Initially they had only three customers, and by the end of the year there were still only nine. Requests for a supply soon came from people outside the estate, and the company obtained a licence from the Board of Trade to increase their area of supply. The initial generating plant was rated at 35kW (47hp), but additional plant was soon added, and by 1890 the generating capacity was 550kW (738hp). TRANSMISSION: AC v DC Except for very small local systems, all supply undertakings had to solve the problems of transmitting electricity at high voltage and then reducing and stabilizing the voltage at a point near the customer. A great rivalry, the ‘battle of the systems’, ensued between the proponents of alternating current and those of direct current. The advantages of AC were that it was easy to change the voltage up and down by means of transformers, and the voltage could be adjusted by tap-changing on the transformers. If, however, it was desired to maintain the supply day and night, then at least one generator had to be kept ELECTRICITY 371 running all the time. With a DC system batteries could be used to maintain the supply at times of low demand. Furthermore, DC generators could easily be operated in parallel: parallel operation of AC machines was always difficult. Another disadvantage with AC, initially, was the lack of a practical electric motor, but after induction motors were developed around 1890, AC supplies became attractive to potential industrial customers. The leading pioneer of AC electricity supply was Sebastian Ziani de Ferranti, a Liverpudlian of Italian extraction. At the age of 17 he was working for Siemens Brothers at Charlton, but he soon branched out on his own. His first company, Ferranti, Thomson and Ince Ltd, was formed in September 1882, with Alfred Thomson, another engineer, and Francis Ince, a lawyer, to manufacture generators; the following year the firm was dissolved and Ferranti, still only 19 years old, set up in business on his own, manufacturing generators, meters and other equipment in London. Within a few years he was chief engineer of the Grosvenor Gallery Company, which had sought his help with technical problems. This company had been formed by Sir Coutts Lindsay to light his art gallery in New Bond Street. Neighbours had been impressed and sought a supply, with the result that the company was soon supplying electricity over a substantial area of Westminster and surrounding districts. They employed the Gaulard and Gibbs distribution system in which electricity was distributed at high voltage to transformers (known as ‘secondary generators’) at or near to each customer. The primary windings of all the transformers were in series and the current in the system was maintained constant at 10 amps. Individual loads were fed from secondary windings on the transformers, and the transformer design was such that the secondary voltages were roughly constant whatever the load. As the Grosvenor Gallery Company’s network expanded, every additional secondary generator required an increase in the circuit voltage, and the practical limit was soon reached. When Ferranti took charge he rearranged the system for parallel working, using distribution at 2400 volts and a transformer on each consumer’s premises to reduce the voltage to 100. He also replaced the Siemens generators originally used by machines of his own design, each able to supply 10,000 lamps of 10 candle power, which required about 350kW. The supply was not metered: each customer paid £1 per 10 candle power lamp per year. As business expanded still further a new company, the London Electric Supply Corporation Ltd, was formed in 1887 with a capital of one million pounds. Ferranti planned a massive power station on the banks of the River Thames at Deptford. Land there was relatively cheap; there was easy access for coal barges, and there was ample cooling water. He designed generators of 10,000hp (7460kW) and planned to transmit electricity into London at 10,000 volts. Such a pressure was quite unprecedented, and he had to design everything himself, including the cables. When the Board of Trade questioned . to start the PART TWO: POWER AND ENGINEERING 364 arc. The rate of fall of the upper carbon was controlled by a brake, and the second function of the mechanism was to control the brake. The smooth operation. direct current. The advantages of AC were that it was easy to change the voltage up and down by means of transformers, and the voltage could be adjusted by tap-changing on the transformers. If,. Light Corporation. The manufacture of Swan’s first commercial lamps was a complex enterprise. The ladies of the Swan household in Newcastle upon Tyne prepared the filaments and Swan himself carbonized them. The