OPEN-WIRE TELEPHONE LINES 



737 



a study has been made of the Weather Bureau records for a number of 

 representative cities in various parts of the country. The chief in- 

 terest naturally centers in the extreme temperatures reached by the 

 wires. It appears that on the average the air temperature will not 

 drop below about — 20 deg. cent. (—4 deg. fahr.) on more than about 

 10 days per year in the colder sections of the country, while a limiting 

 temperature of about 35 deg. cent. (95 deg. fahr.) will not be exceeded 

 on more than about 10 days per year in the warmer sections of the 

 country. Because of imperfect radiation, the temperature of a wire in 



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a. 

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Z 0.004 



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o 



LJ 

 LJ 



a. 

 o 



u 



'^ 0.003 



z 



LJ 



^ 0.002 



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0.001 



100 



500 



1000 5000 10,000 



FREQUENCY — CYCLES PER SECOND 



50,000 100,000 



Fig. 6 — A.-C. temperature coefficient or resistance for open-wire pairs at 20 deg. cent. 



the sun will ordinarily exceed the temperature of the surrounding air 

 by a small amount. A few tests have indicated that for open wires the 

 increase over the air temperature on a warm day is not more than 5 deg. 

 cent. Temperatures of — 20 deg. cent. (— 4 deg. fahr.) and 40 deg. 

 cent. (104 deg. fahr.), therefore, appear to be representative values 

 for the limiting temperatures assumed by open-wire lines. Reference 

 to Equation (1) shows that this range of temperature gives possible 

 variations in the d.-c. resistance of 16 per cent below and 8 per cent 

 above the value for 20 deg. cent. 



The total annual change in resistance at any one place will, of course, 

 be less than the sum of the above changes. In the Middle West, how- 

 ever, where the weather variations are much greater than in other parts 

 of the country, the total annual change in d.-c. resistance may be as 



