An Encyclopedia of the History of Technology part 39. This one of a kind encyclopedia presents the entire field of technology from rudimentary agricultural tools to communication satellites in this first of its kind reference source. Following an introduction that discusses basic tools, devices, and mechanisms, the chapters are grouped into five parts that provide detailed information on materials, power and engineering, transportation, communication and calculation, and technology and society, revealing how different technologies have together evolved to produce enormous changes in the course of history | PART TWO POWER AND ENGINEERING 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 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 .
