166 



Introduction to the Study of Science 



-/ VACUUM. 



STRONG TIP 

 TO PREVENT 

 TIP BREAKAGE 



The conducting wires through the glass stem are of platinum. 

 Platinum is the only metal that expands and contracts under 

 the influence of heat at the same rate as does glass. If a metal 

 with a different rate of expansion were 

 used, it would cause leakage or breakage 

 between the wires and the glass. Examine 

 the way in which platinum wires are con- 

 nected with the brass base, and the way 

 in which the connection is made with the 

 service wires through the receiving socket. 

 Comparison of carbon and tungsten fila- 

 ments. The diameters and lengths of 

 carbon and tungsten filaments are very 

 different. The carbon is thick-and rela- 

 tively short, perhaps only one eighth as 

 long as the tungsten filament. The tung- 

 sten is much finer and may be a yard or 

 more in length. Why are there these 

 differences ? 



Let us consider first a few facts about the metal tungsten. 

 It is a comparatively rare metal, found chiefly in mines in 

 Colorado, Montana, and California. The market quotations 

 show that it is a valuable ore, worth about $6.50 a pound. It 

 is one of the heaviest and hardest of metals, being hard enough 

 to scratch glass. What precious stone does this? Of what 

 are glass cutters made? The melting point of tungsten, or 

 rather the temperature at which it vaporizes, is very high ; 

 the tungsten filament vaporizes at about 3200 C., or 5792 F. 

 It does not melt as ice or iron, but vaporizes directly. In this 

 respect it is like carbon, which in the arc lamp and in the 

 incandescent lamp vaporizes at high temperatures. The 

 black in an incandescent bulb is due to the vapor from the 

 filament. 



The temperature of the tungsten filament may be 250 higher 

 than that of the carbon filament and not vaporize. Because of 



FIG. 47. The struc- 

 ture of a tungsten filament 

 incandescent lamp. 



