THE TANTALUM LAMP. 



187 



at 110 volts with reference to its various periods of life is shown in 

 the following table : 



The initial increase of illuminating value and of current consumed 

 is doubtless caused by a change in the structure of the tantalum 

 wire, this change being ac- 

 companied by a reduction 

 of resistance and, conse- 

 quently, of the phenomena 

 resulting therefrom. \\'v 

 may say at once that aftei- 

 a certain amount of use the 

 filament presents a radical 

 change in appearance when 

 viewed with the naked eye. 

 Wliile the fresh filament 

 has a perfectly smooth and 

 cylindrical surface, it ac- 

 quires a peculiarly glisten- 

 ing aspect as it grows old, 

 so that a lamp having 

 served for some time can 

 be readily distinguished 

 from a new lamp. ^Mien 

 looked at under the microscope, the filament that has burned for a 

 length of time shows a clear tendency toward contraction and forma- 

 tion of drops or beads. Figure 5 is an illustration of a piece of 

 filament in its fresh state and of the same piece after 1,000 hours of 

 service, the specimen in each case being magnified one hundred times. 

 This gradual shortening of the filament can also be observed in the 

 lamps themselves, and offers a further indication of the age of a lamp. 

 Figure G represents the filament frame of a new lamp. It will be 

 noticed that the tantalum wire is led up and down and hangs loose 

 on the supporting frame in easy wide arches, without sharp bends. 

 SM 1905 13 



Fig. 5.— Tantalum iilament, 

 before and after 1,000 

 hours' use. 



Fig. 6— Filament frame 

 of a new lamp. 



