552 Resistance and Inductance of a Concentric Main. 



If the main consisted o£ a hollow inner conductor whose 

 radii were 1 and ^/2 respectively and an outer conductor 

 of radii 2' 6 and 2*4, then we would have 



p /, 0'105m 4 \ p/, O0072??i 4 \ 



Hence, making the inner conductor hollow has appreciably 

 reduced the losses due to skin resistance. It will also be 

 noticed that the losses due to this cause in the (1, \/2) 

 cylindrical conductor are about 30 per cent, less when the 

 return current is outside it than when it is inside it. 



It is easy to see that i£ the current returns by a fine con- 

 ductor in a minute hole along the axis of a cylindrical 

 conductor, the increase in the losses for low-frequency currents 

 will be nearly seven times greater than if it returned by a 

 concentric tube outside the conductor. 



IX. Values of m for Copper Conductors. 



The volume resistivity of high conductivity annealed 

 copper at 60° F. (15°'6 C.) is 1696-5. Hence 



2 _ knjiw _ _8ttV * 

 p ~ 1696-5 7 ' 



Assuming that /a=1, we get the following table : — 



/. 



m. 



/• 



m. 



5 



0-4824 



100 



2-1573 



10 



0-6822 



200 



3-0509 



15 



0-8355 



300 



3-7366 



20 



0-9648 



400 



4-3147 



25 



1-0787 



500 



4-8240 



30 



1-1816 



1000 



6-8221 



35 



1-2763 



5000 



15-255 



40 



1-3644 



10000 



21-573 



45 



1-4472 



100000 



68-221 



50 



1-5255 



1000000 



1 



215-734 



