CHAPTER IV. 



ABSORPTION SPECTRA OF AQUEOUS SOLUTIONS OF CERTAIN 

 SALTS OF NEODYMIUM AS STUDIED BY MEANS OF 

 THE RADIOMICROMETER. 



The radiomicrometer is simply a thermoelement attached to a loop of 

 thin copper wire suspended in a magnetic field. One of the greatest diffi- 

 culties in constructing this element is to obtain copper wire free from all 

 magnetic metals. If perfectly pure copper wire could be found, an instru- 

 ment could be constructed of almost any desired sensibility. 



A very good specimen of small copper wire was furnished us by Leeds and 

 Northrup, of Philadelphia. This wire was dipped in dilute nitric acid and 

 the exterior dissolved away until the wire was of proper size. The removal of 

 the outside coating of the wire removed practically all of the magnetic mate- 

 rial from it, this material probably being iron from the dies through which 

 the wire was drawn. 



It was not a simple matter to construct a satisfactory thermo-electric 

 junction. The alloys used in making this junction were 90 parts bismuth 

 and 10 parts tin, and 97 parts bismuth and 3 parts antimony. The method 

 of making the thermo-electric junction and of soldering it on to the ends of 

 the loop of copper wire we owe to Professor A. H. Pfund. 1 Fine strips of 

 the alloys were obtained in the following manner: 



A few grams of the alloy in question were fused in a vessel free from all 

 magnetic material, and then thrown tangentially upon a clean and smooth 

 glass plate. In this way strips of the metal were obtained of almost any 

 desired thickness. Some were too thin to handle, those used being about 

 1 mm. wide, 0.01 mm. thick, and about 5 mm. in length. 



The thermo-element was made by soldering an end of a strip of one of the 

 above-named alloys to an end of a strip of the other, the whole having the 

 form of a letter V. The two free ends of the V were soldered to the two ends 

 of the loop of copper wire. The soldered surfaces were blackened to absorb 

 the energy more completely. At the end of the loop of copper wire opposite 

 the thermo-element a light glass rod is fastened. This carries the mirror 

 and is suspended from above by a quartz fiber. The mirror employed was 

 about 4 sq. mm. This entire system, consisting of thermo-element, loop of 

 copper wire, and mirror, weighed about 20 mg. It was suspended by means 

 of a quartz fiber so that the loop hung between the poles of a strong magnet. 



1 Plws. Rev., 34, 22S (1012). Phys. Zeit., IS, S70 (1912). 



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