CHAPTER II 



WIRES, WIRE TABLES, RESISTIVITY AND TEMPERATURE 

 COEFFICIENT 



Note. Unless otherwise stated the resistivity of copper at 

 atmospheric temperature may be taken as 11 (based on the 

 circular mil-foot). 



Note also that in the American, or Brown & Sharp wire gauge 

 the diameters of the wires from No. 6 to No. 12 are approximately 

 the reciprocals of the numbers expressed in inches; thus No. 10 is 

 T V inch or 100 mils diameter (actual 102). Also that the diameter 

 doubles for every 6 numbers and the area for every 3 numbers, 

 and the area increases 10 times for 10 numbers. Use the B. & S. 

 wire table on page 103. 



1. By referring to the diameters of the sizes from 6 to 12, ob- 

 tained without consulting the tables, determine the approximate 

 diameter in mils and the area in circular mils of the following wires: 

 Nos. 35, 23, 5, 1. Determine also the per cent errors in area that 

 would be made in using these approximations. (See wire table 

 for exact sizes.) (15 min.) 



2. The area of a No. 10 wire being 10,400, what will be the 

 approximate area of a No. 4 wire? Of a No. 13? Of a No. 20? 

 (2 min.) 



3. If one dimension of a rectangular wire is to be twice the 

 other, what must they be in inches to replace a No. 6 wire? What 

 will be the area in sq. mm.? (4 min.) 



4. How many square mils and how many circular mils in a 

 wire J inch by T 3 ^ inch? Also if this wire is rounded at the cor- 

 ners with a radius of 20 mils, what will be its area in circular 

 mils? (4 min.) 



5. Without consulting the tables determine what B. & S. wires 

 will have to be used in circuits requiring the following areas of 

 copper: 40,000, 3600 and 100 circular mils. Also for the following 

 diameters in mils: 7.5, 19, 62, 135 and 240. (6 min.) 



6. Given a copper wire 300 feet long and 6529 circular mils 

 cross-section, No. 12 B. & S., find the volts to give 25 amperes. 

 Also if the length be made 600 feet. (2 min.) 



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