384 BELL SYSTEM TECHNICAL JOURNAL 



that the wire on one spool also contained thallium and this wire was 

 rejected for further use as contact material. Total time spent: 

 three hours. 



A lot of zinc straps for supporting aerial cables was rejected in the 

 field because the straps were "brittle," i.e., when flexed they would 

 crack rather than bend freely. Between areas which cracked there 

 were areas which bent easily and showed no signs of breaking. Spec- 

 trochemical examination of the zinc at the point of fracture revealed 

 the presence of mercury, while this element was absent in the flexible 

 areas (Fig. 1). Time spent: two hours. 



2. Quantitative Analysis by Estimation 



The first question usually asked when a qualitative analysis shows 

 the presence of an element is, "How much is present?" It is easy to 

 to tell from a qualitative plate whether the element in question is 

 present as a major component, a minor component, an impurity, or 

 merely as a trace. This information is frequently the deciding factor 

 as regards further analytical work. This is particularly important in 

 diagnosing troubles in field complaints. Very often the spectro- 

 chemical evidence alone will settle the matter — if not it will almost 

 invariably tell just what further analyses are necessary, thus promoting 

 the greatest possible efficiency in reaching a final solution of the 

 problem. 



The method of making such a rough quantitative analysis is to rely 

 upon the experience of the analyst in judging the percentage of the 

 element necessary to give the observed density of the lines under the 

 conditions used. While such a procedure is theoretically unsound, an 

 experienced analyst can make a surprisingly accurate "guess" by 

 this method as has been repeatedly demonstrated by subsequent 

 quantitative analysis. 



An excellent example of an application of this technique of estima- 

 tion is the identification of contact materials. It is sometimes desir- 

 able to identify the material composing a contact without removing 

 the apparatus from its mounting, without impairing the contact for 

 further use, and sometimes without more than momentarily interrupt- 

 ing its operation. The method used is to rub the surface of the contact 

 with a small piece of fine, especially pure abrasive paper and burn the 

 paper in a graphite arc, photographing the resulting spectrum. From 

 the lines appearing in this spectrum which do not appear in the spec- 

 trum of the abrasive paper alone the elements composing the contact 

 can be identified and their proportions estimated (Fig. 2). 



