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h still subject to many improvements and capable of great enlargement； and soon Edison。 sold out his interests in it and turned his great mind to other inventions。
The inventive ingenuity of others brought in time better and more economical incandescent lamps。 From the filaments of bamboo fiber the next step was to filaments of cellulose in the form of cotton； duly prepared and carbonized。 Later （1905） came the metalized carbon filament and finally the employment of tantalum or tungsten。 The tungsten lamps first made were very delicate； and it was not until W。 D。 Coolidge； in the research laboratories of the General Electric Company at Schenectady； invented a process for producing ductile tungsten that they became available for general use。
The dynamo and the central power station brought the electric motor into action。 The dynamo and the motor do precisely opposite things。 The dynamo converts mechanical energy into electric energy。 The motor transforms electric energy into mechanical energy。 But the two work in partnership and without the dynamo to manufacture the power the motor could not thrive。 Moreover； the central station was needed to distribute the power for transportation as well as for lighting。
The first motors to use Edison station current were designed by Frank J。 Sprague； a graduate of the Naval Academy； who had worked with Edison； as have many of the foremost electrical engineers of America and Europe。 These small motors possessed several advantages over the big steam engine。 They ran smoothly and noiselessly on account of the absence of reciprocating parts。 They consumed current only when in use。 They could be installed and connected with a minimum of trouble and expense。 They emitted neither smell nor smoke。 Edison built an experimental electric railway line at Menlo Park in 1880 and proved its practicability。 Meanwhile； however； as he worked on his motors and dynamos； he was anticipated by others in some of his inventions。 It would not be fair to say that Edison and Sprague alone developed the electric railway； for there were several others who made important contributions。 Stephen D。 Field of Stockbridge； Massachusetts； had a patent which the Edison interests found it necessary to acquire； C。 J。 Van Depoele and Leo Daft made important contributions to the trolley system。 In Cleveland in 1884 an electric railway on a small scale was opened to the public。 But Sprague's first electric railway； built at Richmond； Virginia； in 1887； as a complete system； is generally hailed as the true pioneer of electric transportation in the United States。 Thereafter the electric railway spread quickly over the land； obliterating the old horsecars and greatly enlarging the circumference of the city。 Moreover； on the steam roads； at all the great terminals； and wherever there were tunnels to be passed through； the old giant steam engine in time yielded place to the electric motor。
The application of the electric motor to the 〃vertical railway；〃 or elevator； made possible the steel skyscraper。 The elevator； of course； is an old device。 It was improved and developed in America by Elisha Graves Otis； an inventor who lived and died before the Civil War and whose sons afterward erected a great business on foundations laid by him。 The first Otis elevators were moved by steam or hydraulic power。 They were slow； noisy； and difficult of control。 After the electric motor came in； the elevator soon changed its character and adapted itself to the imperative demands of the towering； skeleton…framed buildings which were rising in every city。
Edison； already famous as 〃the Wizard of Menlo Park；〃 established his factories and laboratories at West Orange； New Jersey； in 1887； whence he has since sent forth a constant stream of inventions； some new and startling； others improvements on old devices。 The achievements of several other inventors in the electrical field have been only less noteworthy than his。 The new profession of electrical engineering called to its service great numbers of able men。 Manufacturers of electrical machinery established research departments and employed inventors。 The times had indeed changed since the day when Morse； as a student at Yale College； chose art instead of electricity as his calling； because electricity afforded him no means of livelihood。
From Edison's plant in 1903 came a new type of the storage battery； which he afterwards improved。 The storage battery； as every one knows； is used in the propulsion of electric vehicles and boats； in the operation of block…signals； in the lighting of trains； and in the ignition and starting of gasoline engines。 As an adjunct of the gas…driven automobile； it renders the starting of the engine independent of muscle and so makes possible the general use of the automobile by women as well as men。
The dynamo brought into service not only light and power but heat； and the electric furnace in turn gave rise to several great metallurgical and chemical industries。 Elihu Thomson's process of welding by means of the arc furnace found wide and varied applications。 The commercial production of aluminum is due to the electric furnace and dates from 1886。 It was in that year that H。 Y。 Castner of New York and C。 M。 Hall of Pittsburgh both invented the methods of manufacture which gave to the world the new metal； malleable and ductile； exceedingly light； and capable of a thousand uses。 Carborundum is another product of the electric furnace。 It was the invention of Edward B。 Acheson； a graduate of the Edison laboratories。 Acheson； in 1891； was trying to make artificial diamonds and produced instead the more useful carborundum； as well as the Acheson graphite； which at once found its place in industry。 Another valuable product of the electric furnace was the calcium carbide first produced in 1892 by Thomas L。 Wilson of Spray； North Carolina。 This calcium carbide is the basis of acetylene gas； a powerful illuminant； and it is widely used in metallurgy； for welding and other purposes。
At the same time with these developments the value of the alternating current came to be recognized。 The transformer； an instrument developed on foundations laid by Henry and Faraday； made it possible to transmit electrical energy over great distances with little loss of power。 Alternating currents were transformed by means of this instrument at the source； and were again converted at the point of use to a lower and convenient potential for local distribution and consumption。 The first extensive use of the alternating current was in arc lighting； where the higher potentials could be employed on series lamps。 Perhaps the chief American inventor in the domain of the alternating current is Elihu Thomson； who began his useful career as Professor of Chemistry and Mechanics in the Central High School of Philadelphia。 Another great protagonist of the alternating current was George Westinghouse； who was quite as much an improver and inventor as a manufacturer of machinery。 Two other inventors； at least； should not be forgotten in this connection： Nicola Tesla and Charles S。 Bradley。 Both of them had worked for Edison。
The turbine （from the Latin turbo； meaning a whirlwind） is the name of the motor which drives the great dynamos for the generation of electric energy。 It may be either a steam turbine or a water turbine。 The steam turbine of Curtis or Parsons is today the prevailing engine。 But the development of hydro…electric power has already gone far。 It is estimated that the electric energy produced in the United States by the utilization of water powers every year equals the power product of forty million tons of coal； or about one…tenth of the coal which is consumed in the production of steam。 Yet hydro…electricity is said to be only in its beginnings； for not more than a tenth of the readily available water power of the country is actually in use。
The first commercial hydro…station for the transmission of power in America was established in 1891 at Telluride； Colorado。 It was practically duplicated in the following year at Brodie； Colorado。 The motors and generators for these stations came from the Westinghouse plant in Pittsburgh； and Westinghouse also supplied the turbo…generators which inaugurated； in 1895； the delivery of power from Niagara Falls。
CHAPTER X。 THE CONQUEST OF THE AIR
The most popular man in Europe in the year 1783 was still the United States Minister to France。 The figure of plain Benjamin Franklin； his broad head； with the calm； shrewd eyes peering through the bifocals of his own invention； invested with a halo of great learning and fame； entirely captivated the people's imagination。
As one of the American Commissioners busy with the extraordinary problems of the Peace； Franklin might have been supposed too occupied for excursions into the paths of science and philosophy。 But the spaciousness and orderly furnishing of his mind provided that no pursuit of knowledge should be a digression for him。 So we find him； naturally； leaving his desk on several days of that summer and autumn and posting off to watch the trials of a new invention； nothing less indeed than a ship to ride the air。 He found time also to describe the new invention in letters to his friends in diffe