HEAT AND LIGHT FROM ELECTRIC CURRENTS 213 



are operated by a motor so that groups of lamps are turned 

 on at short intervals, giving many marvelous and often 

 deceptive effects. Almost every journey into one of the 

 main streets of a large city discovers new devices for using 

 electricity in advertising. 



223. Other devices utilizing 

 heating effects. There are 

 many appliances which utilize 

 the heating effect of an elec- 

 tric current. Probably the 

 most important of these is 

 the electric lamp. The first 

 incandescent lamps were made 

 in 1879 by Thomas A. Edison. 

 The electric current was 

 passed through a piece of 

 charred bamboo fiber, which 

 was placed in a vacuum. This 

 filament was used until about 

 1894, when a process for im- 

 proving the carbon was dis- 

 covered. The metal filament 

 lamps were introduced about 

 1906 and are still undergoing 

 improvement. N 



The carbon lamp is rapidly 

 going out of use. The im- 

 proved carbon lamp, with 

 metallized filament, is used 

 to some extent where low first cost is considered. The tanta- 

 lum lamp has been displaced by the tungsten lamp, and 80 

 per cent of the lamps now used are said to be of the tungsten 

 type. The vacuum lamp, in sizes below 100 watts, is used 

 in electric signs, automobiles, flashlights, electric cars, and 

 railway coaches. The gas-filled tungsten lamp is used for 



FIG. 105. An ornamental lamp post 



Note the lead-covered conduit through 



which the electric wires lead to the 



tungsten lamp 



