502 BELL SYSTEM TECHNICAL JOURNAL 



tion between very thin layers of a material. In a typical case, sheets 

 of an iron-cobalt magnetic alloy were observed to undergo a change 

 in properties after rolling to diaphragm thickness. It was suspected 

 that the surface had lost iron through oxidation and subsequent 

 mechanical removal. By using pure silica abrasive, a layer of metal 

 was removed from the surface corresponding to a thickness of about 

 0.005 millimeter. The metal was then extracted from the abrasive 

 by acid treatment and the iron-cobalt ratio determined microchemi- 

 cally. A similar technique has been applied to tinned copper wire, to 

 determine the quantity of copper dissolved by tin during the tinning 

 process and the extent of its migration to the surface of the coating. 

 It has also been used to study the alloy layer formed between the zinc 

 coating and the iron base in sherardizing and galvanizing processes. 

 Micromethods applied to thin films have been used in the study of 

 the copper-oxygen ratio variation in copper oxide rectifier discs. 



The inclusion of foreign particles in the surfaces of metals and other 

 materials occasionally occurs as a result of faulty conditions of manu- 

 facture. Determination of their nature and source is naturally 

 necessary before remedial measures can be taken. Since the particles 

 are small, frequently even of microscopic dimensions, microchemical 

 methods in such cases are the only ones practicable. Thus small 

 hard particles were observed in an experimental lead cable alloy which 

 were at first thought to consist of segregated impurities. Micro- 

 analysis- showed these particles to be composed of iron and nickel, 

 indicating that they had probably been accidentally introduced into 

 the surface during the extrusion process. Paper removed from a 

 condenser that had failed on test was found to contain microscopic 

 particles of iron, iron rust, brass and carbonaceous aggregates which 

 had also been accidentally incorporated during manufacture. 



The isolation, detection and determination of traces of impurities 

 or substances otherwise associated with large quantities of a given 

 material have benefited through the application of microtechnique. 

 Methods have been worked out for determining sulphur and phos- 

 phorus in steel and for silver in lead in which these impurities are all 

 present in quantities less than 0.001 per cent. Acetic acid has been 

 isolated from the corrosion products occurring when lead cable is 

 exposed in creosoted wood ducts. The quantity actually present is 

 usually very small, amounting to between 0.01 per cent and 0.001 

 per cent of the weight of the corrosion product. 



Electrolytic corrosion in the windings of relays and other similar 

 types of apparatus may occur as the result of minute quantities of 

 salts present in the insulating materials or acquired from the manu- 



