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THE BELL SYSTEM TECHNICAL JOURNAL, JULY 1951 



method. Variations of weight and dimensions along individual lengths of 

 either wire or tubing are not significant as determined by checks at intervals 

 on a number of the lengths. 



The measurements of d-c. resistance needed to determine the resistivity 

 of the copper conductors are subject to limitations of knowledge of tempera- 

 ture, particularly where the samples must be measured in long straight 

 lengths. The laboratory setup used to obtain the data consisted of a long 

 trough through which oil was circulated over the immersed samples. A 

 feature was the use of a 200 gal. tank of water in thermal contact with the 

 oil circuit to maintain it at a constant, or very slowly changing temperature. 



Table II 

 Measured Average Physical Constants of Copper Tubing and Wire for Coaxial 



Impedance Standards 



Density (gm/cc) 



Outside Diameter (inches) 



Inside Diameter (inches) 



Wall Thickness (inches) 



Volume Resistivity (microhm cm) 



Mass Conductivity (per cent of lACS) 



Temperature Coefficient of Resistance, 20°C. 



Wire 



Tubing 



Table III 

 Measured Average Physical Constants of Expanded Polystyrene Insulating 

 Cylinders for Coaxial Standards 



Volume Expansion Ratio. 



Weight (gm/disc) 



Length (inches) 



Density (gm/cc) 



Dielectric Constant 



41 

 0.1789 

 0.396 

 0.0257 

 1.033 



A Kelvin double bridge used for the tests was operated in accordance with 

 minimum-error principles.'* 



Components-Wire 



The center conductor of each coaxial must be drawn staright so that only 

 light spaced support need be used to keep it in axial alignment with the 

 tube. Since available commercial wire drawing machines normally depend 

 on a driven small-diameter capstan to pull the wire through the final re- 

 ducing die, it was necessary to draw the wire in the laboratory so that 

 straight-out drawing could be achieved. Commercial machines were, how- 

 ever, used to reduce the supply from f" rod to 0.110" dia. wire without 



* Electrical Measurements, Laws, McGraw-Hill, 1917. 



