THE FIRST ELECTRIC LAMP 



247 



Suspend a ten-foot length of fine iron wire (No. 26) from insulated 

 supports. Fasten one wire of a no- volt circuit to one end of the 

 iron wire, and bring the other end of this circuit in contact with the 

 other end of the wire. Move the end of the circuit wire along, shorten- 

 ing the length of the iron wire until it becomes red hot. All this heat 

 energy is produced at the expense of electrical energy. As the wire is 

 shortened the heat increases. The hotter the wire, the greater per cent 

 of electrical energy is changed to heat. 



COST OF ELECTRIC LIGHT 



4O Woctt rToryla -D Lamp 1907 - 1913 

 1000 Woctt ITajdUx- C Lamp 1914- -1920 



Graph showing decrease in cost of electric lighting in forty years. A is the reduction 

 due to lamp improvement only ; B, due to lamp improvement and reduced rates 

 for current. 



We thus see from the result of tbis experiment that it is 

 more economical to use electric bulbs which contain fila- 

 ments which can be raised to high temperatures. Why? 



Experiment. To show the advantage of tungsten over carbon lamps. 



Connect one or more small carbon lamps in a circuit with a kilowatt- 

 hour meter. Find the electrical energy used in running these lamps 

 for a definite time, say one hour. Replace the lamps by an equal 

 number of small tungsten lamps (25 watt), and find the electrical 

 energy consumed during an equal interval of time. 



Each carbon lamp gives 16 candle power, and each tungsten lamp gives 

 20 candle power. Reckoned on the basis of candle power, what is 

 the ratio of cost of running the two types of lamps ? 



