NO. 2 HISTORY OF ELECTRIC LIGHT SCHROEDER 53 



in comparison with the modern lOO-watt gas-filled tungsten-filament 

 lamp which has an initial efficiency of 12.9, and a mean efficiency of 

 II. 8, 1-p-w. In other words the equivalent (wattage) size of modern 

 lamp gives over seven times when new, and eleven times on the 

 average, as much light for the same energy consumption as Edison's 

 first commercial lamp. In the latter part of 1881 the efficiency was 

 changed to " ten lamps per horsepower," equivalent to 2I 1-p-w 

 initially. Two sizes of lamps were made: 16 cp for use on iio-volt 



( ) 



^j^^^ 



Final Form of Screw Base, 1881. 



With plaster of paris, the previous form of base was apt to pull 

 apart when the lamp was firmly screwed into the socket. The form 

 of the base was therefore changed to that shown, which overcame 

 these difficulties, and which has been used ever since. The lamp 

 shown was standard for three years and is in the exhibit of Edison 

 lamps in the Smithsonian Institution. 



circuits and 8 cp for use either direct or 55 volts or two in series on 

 1 10- volt circuits. 



EDISOn's THREE-WIRE SYSTEM 



The distance at which current can be economically delivered at 

 no volts pressure is limited, as will be seen from a study of Ohm's 

 law. The loss of power in the distributing wires is proportional to 

 the square of the current flowing. If the voltage be doubled, the 

 amount of current is halved, for a given amount of electric power 

 delivered, so that the size of the distributing wires can then be reduced 

 to one-quarter for a given loss in them. At that time (1881) it was 



