ELECTRIC DISTRIBUTION AND WIRING. 



303 



Therefore, using the value of k from (iv), and neglecting k\(d J?) which is very 



small in comparison with k\R, we have : 



E 



/max 



Iog 10 - 



(49) 



in which /max is the maximum electric stress (volts per centimeter) in the medium 

 between two parallel wires each of radius R centimeters, E is the electromotive force 

 between the wires in volts, d is the distance in centimeters between the axes of the 

 wires, and Log e d\R is the Naperian logarithm of the ratio, d\R. 



Example. The electrical strength of air is about 26, ooo volts per centimeter.* 

 Therefore, substituting 26,000 for/ max in equation (49), we find that 3,014 volts is 

 the greatest electromotive force that the air can sustain between two wires 6 mils in 

 diameter and 6 inches apart; and that 101,800 volts is the greatest electromotive 

 force that the air can sustain between two wires 500 mils in diameter and 6 inches 

 apart center to center. It is worthy of note that when the voltage between two very 

 fine wires but slightly exceeds the sustaining strength of the intervening air, the air 

 breaks down only in the immediate neighborhood of the wires, and discharge from 

 wire to wire takes place mainly by convection currents of electrically charged air. 



1600 



1200 



The dependence of the safe value of voltage between wires upon the size of the 

 wires is shown in a very interesting way by the curves f in Fig. 173. Curve A shows 



* Dielectric Strength of Air, by C. P. Steinmetz, Transactions A. I. E. E., Vol. 

 XV., pp. 281-326, 1898. 



f From a paper by Charles F. Scott on High-Voltage Power Transmission, 

 Transactions A. I. E. E., Vol. XV., pp. 531-576. 



