138 



THE TANTALUM LAMP. 



But after being used for some time the aspect of the lamp is quite 

 different. As shown in figure 7, the wire has contracted, the wide 

 arches have disappeared and sharp-pointed angles have taken their 

 place. 



The behavior of these lamps is most peculiar when the filament 

 lias burnt through. While with all other incandescent lamps the 

 burning through of the filament is tantamount to the economical 

 death of the lamp, it may happen with tantalum lamps that the^^ 

 burn through several times without being rendered useless; on the 

 contrary, each burning through is followed by an increase, often 

 considerable, of the illuminating power. This peculiar result is 

 due to the fact that in many cases a broken wire comes in contact 

 with its neighbor, so that the circuit is again established. A part 



Fig. 7. — Appearance of tilameut 

 after having been in use. 



Fig. 8. — Filament frame, showing 

 broken filament. 



of the filament is thus cut out of the circuit, and the lamp conse- 

 quently burns more intensely, and sometimes even too intensely, in 

 which case, of course, only a short span of life is left to it. Yet we 

 have had more than one lamp under observation, the filament of 

 which broke for a first time after a short period of service and then 

 broke repeatedly, but notwithstanding this the lamp lived more than 

 1,000 hours. We have often succeeded in rendering a lamp with a 

 broken filament serviceable again by tapping it to bring the broken 

 piece into contact with its neighbor. Figure 8 represents the frame 

 of a lamp in which the filament was burnt through in three places, 

 and yet continued to do service. For the sake of clearness, the 

 l)ack spans of the filament have been omitted in the drawing, while 

 the front spans which were carrying the current are drawn in 

 specially heavy lines. 



