Artificial Lighting 167 



this higher temperature it produces more light. This accounts 

 for the high luminous efficiency of the tungsten lamp ; the more 

 intense the heat of the filament without vaporizing, the brighter 

 or more incandescent will be the light. 



65. Conductivity of substances. We take up now the study 

 of the fineness and length of the tungsten filament in comparison 

 with the carbon. In the study of heat transfer (pages 94 ff.), it 

 appeared that different substances conduct heat with different 

 degrees of facility. Some metals, such as silver, copper, alu- 

 minum, and iron, are regarded as good conductors ; other sub- 

 stances are medium, and still others, such as wood, glass, paper, 

 liquids, and gases, are so poor as to be considered heat-insulators. 

 In the transmission of the electric current, substances show 

 similar differences as conductors. The wide use of copper, iron, 

 and aluminum wires suggests that these may be good conduc- 

 tors, and exact measurement shows them to be among the best 

 conductors of electricity, as they were found to be of heat. 

 Silver is the best conductor, copper next, and aluminum third. 



The common practice is to speak, not of the facility with 

 which materials conduct, but of the resistance which they 

 offer to the electric current. It is useful to know what are 

 good, indifferent, and bad conductors. For conductors are 

 good or bad according to the amount of resistance they offer 

 to the electric current. There are, moreover, no perfect con- 

 ductors, that is, none that at usual temperatures would have 

 zero resistance. And there are no substances which are per- 

 fect non-conductors or resist completely the passage of elec- 

 tricity. There are substances which transmit very little of the 

 electric current and they are called insulators. Such sub- 

 stances include glass, rubber, mica, paraffin, dry wood, paper, 

 etc., but they are insulators only at usual temperatures and 

 with a certain strength of the electric current. 



A list is given of common substances arranged in accordance 

 with the resistance they offer to electricity. A perfect con- 

 ducting substance would have zero (0) resistance. The others 



