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HANDBOOK OF PHOTOGRAPHY 



to air are frequently seen in spark spectrograms; these can usually be diminished in 

 strength or eliminated entirely by inserting a coil of a few dozen turns of wire, with 

 iron core, as shown in Fig. 13. The number of turns should be adjusted so that the air 

 lines are reduced as much as possible without too greatly reducing the intensities of the 

 lines which are being studied. 



Advantages of the arc compared to the spark are that it is safer electrically, is more 

 intense, is usually quieter, produces no air lines, and shows greater variation between 

 lines so that they can be identified more readily by their appearance. Advantages 



Variable 

 reactor 



Fig. 13. — Electrical circuit of spark light source for use in spectroscopy. 



of the spark are that it Isurns less material, needs less attention, brings out some lines 

 not found in the arc, and produces fewer band lines. The cyanogen bands are par- 

 ticularljr annoying in the violet and near ultraviolet regions when a graphite arc is 

 burned in air. Burning metallic material in the arc Aveakens these bands. They 

 produce a strong violet light in the arc, and by carefully watching it and refilling with 

 material when this color appears, the intensity of the bands can be reduced. 



There is no royal rule as to when an arc is to be preferred to a spark, or vice versa. 

 Most spectroscopists keep both available and use whichever is best for the problem 



Fig. 14. — Spark A and arc B spectrograms made under identical conditions. 



in hand. Figure 14 shows spectrograms taken under identical circumstances of the 

 same material, A being a spark exposure, and B an arc exposure. 



High-frequency arcs, high-voltage arcs, vacuum arcs and sparks, electrodeless 

 discharges, and nimierous other light sources are often found useful for spectroscopic 

 purposes and will be found described in the literature. 



Light Sources for Absorption Spectroscopy. — When transparent liquids or solids 

 are studied by absorption spectrophotometry, new absorption bands can be located 

 most readUj'" if light producing a continuous spectrum is sent through the absorbing 

 material. The most convenient source for this purpose between 10,000 and 4000 A. is 

 a straight-filament incandescent lamp, while between 6000 and 2000 A. a high-voltage 

 discharge through hydrogen gas in a quartz tube is useful. A high-tension spark 

 discharge under distilled water between tungsten or aluminum electrodes, or electrodes 

 of almost any other metal, gives a useful continuous background throughout the entire 



