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BELL SYSTEM TECHNICAL JOURNAL 



causes the surfaces to become brighter and vice versa. This provides a 

 few thousandths of a per cent latitude in adjusting the sensitive range of the 

 method to the median calcium content desired. Alloys having calcium 

 contents outside the range of the method may be estimated by admixture 

 with known amounts of lead-calcium alloys of known calcium content. 

 Pure lead, in general, cannot be used for dilution because the oxide and 

 possibly traces of impurities present in it cause the loss of some calcium 

 from the mixture. 



To date most experience with the use of the visual test for calcium has 

 been on alloys made with chemical lead. This grade of lead is substantially 

 free from As, Sn, Bi, Fe, Sb and Zn, and contains about 0.004 per cent Ni, 

 0.06 per cent Cu and 0.007 per cent Ag. The variation in concentration of 

 these elements in the commercial supply has not been found to be great 

 enough to cause serious interference with the indications given by the 

 casting test. However, the method is influenced by certain variations in 

 impurity content which are in excess of those normally encountered in the 



CO2 



REMOVER HUMIDIFIER 



H2O OR SALT 

 SOLUTION 

 POTASSIUM 

 HYDROXIDE 



RELEASE 

 VALVE 



DEWAR 

 FLASK 



TO CASTING 

 CHAMBER 



SOLID 

 CO2 



Fig. 3 — Apparatus used for regulating atmosphere composition 



usual supply. The presence of tin in the order of a few thousandths of one 

 per cent causes low indications of calcium content. The use of high purity 

 lead in place of "chemical" lead has a similar effect. Bismuth additions 

 up to 0.1 per cent are inconsequential. Arsenic and antimony form com- 

 pounds with calcium which dross off in the mixing kettle so that these 

 elements would not be found in the finished sheath. The effects of the 

 interfering impurities referred to above can be corrected for, when their 

 presence is known, by varying the atmosphere in the casting chamber or by 

 admixture with a known proportion of alloy having a pre-determined 

 higher calcium content. By these procedures fissuring can be made to 

 occur in alloys that otherwise do not give this manifestation. In practical 

 operation the brand of lead being used will be known and the necessary 

 adjustments can be made in the conditions of the test. 



Sufficient analytical data have been collected to establish the fact that 

 the method here presented is rapid, reliable, and extremely helpful for 

 plant control application. 



