SPECTROSCOPIC PHOTOGRAPHY 821 



spectrum, though this is usually crossed with occasional emission or absorption lines. 

 A condensed spark in air between cadmium electrodes can be made to give a heavy 

 continuous background in the ultraviolet region, though crossed with a number of 

 emission lines. Exploded wires probably give the most intense continuous back- 

 ground, but, compared to the other methods listed, these are difficult to control and 

 inconvenient to use. 



When extinction coefficients are to be measured, especially in the ultraviolet 

 region, a rich line spectrum can be used conveniently for background, since measure- 

 ments must be made at known wavelengths and these need not be continuouslj' spread. 

 A spark between tungsten-steel electrodes is commonly used for this purpose. 

 Uranium electrodes are also useful — in fact, any hard metal with numerous lines in 

 the ultraviolet will serve for this purpose. A spark source is commonly used with 

 the Hilger Spekker Photometer and with rotating-sector disk photometers, which are 

 commonly used for ultraviolet absorption spectrophotometry. 



Bibliography 



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