248 Master Minds of Modern Science 



making an electric lamp suitable for domestic use. Each 

 independently invented a filament that could be heated 

 to incandescence in a vacuum bulb. Both suggested that 

 electric current should be laid on to houses and buildings 

 like water or gas, but Edison was the first actually to do 

 this work. That happened between 1878 and 1880. 



Edison saw that the voltage or pressure must not be 

 too high, and also that it must be possible for each light 

 to be turned off or on without affecting other lights. This 

 meant that the voltage must be somewhere about one 

 hundred and that each lamp must have its terminals 

 connected to two supply- wires. Edison saw also that he 

 must have several dynamos in action, so that all the eggs 

 would not be in one basket. 



In those days, of course, there was no maker of electric- 

 light appliances, and Edison had first to invent each one 

 separately, and then to make it. He selected as his 

 standard pressure a supply of one hundred and ten volts, 

 and designed constant-pressure, shunt-wound dynamos, 

 with drum armatures. In these the field electro-magnet 

 consisted of two massive iron pole pieces at the end of 

 long iron bars, or legs, which were wound with magnetizing 

 coils and connected at the top by an iron yoke. These 

 dynamos were separately driven, but sent their currents 

 into a common pair of electric mains called 'bus bars/ 



In 1879 Edison lighted the streets and some buildings 

 in the suburb of Menlo Park, including his laboratory, 

 office, and three houses. On New Year's Eve, 1879, three 

 thousand people came to see the new lighting. Later 

 Edison equipped a steamship, the Columbia, with about 

 one hundred lamps, and this installation worked well for 

 several months. The lamps withstood a voyage round 

 the Horn to San Francisco, and were inspected with much 

 interest at Rio de Janeiro, Valparaiso, and other ports. 

 But as Edison has since said: "We had a successful 

 lamp, but it was not economic. It was fragile and costly, 



