419 



TABLE 426.— THE CALCULATION OF THE HIGH-FREQUENCY RESISTANCE 

 OF CONDUCTORS (concluded) 



Part 3. — Coefficients in formula for proximity factor of equal parallel 



round wires 



The proximity factor of two equal parallel conductors may be calculated by the formula 

 P = l + {G-dVs^/iFd-Hdys 2 )] 



in which the coefficient F is to be obtained from Part 1 for the given value of ^f/Ro and 

 the coefficients G and H are to be taken from the table below for the given value of V//7? . 

 In the table below the values of H apply to currents in the same direction ; in the case of 

 currents in opposite directions H' is to be used. In the above formula d is the diameter of 

 the wires and j their axial spacing. The proximity factor for two equal parallel tubular 

 conductors does not diff er mu ch from the value for two solid wires with the same axial 

 spacing and a value of V //./?<> one-half the value for two solid wires of the same diameter, 

 except for conductors very close together. 



TABLE 427.— RATIO OF ALTERNATING TO DIRECT CURRENT RESISTANCES 



FOR COPPER WIRES 



This table gives the ratio of the resistance of straight copper wires with alternating cur- 

 rents of different frequencies to the value of the resistance with direct currents. 



Values between 1.000 and 1.001 are indicated by *1.001. 



The values are for wires having an assumed conductivity of 1.60 microhm-cm; for 

 copper wires at room temperatures the values are slightly less than as given in table. 



The change of resistance of wire other than copper (iron wires excepted) may be cal- 

 culated from the above table by taking it as proportional to dVf/p where d = diameter, 

 / the frequency (cycles/sec) and p the resistivity. 



If a given wire be wound into a solenoid, its resistance, at a given frequency, will be 

 greater than the values in the table, which apply to straight wires only. The resistance in 

 this case is a complicated function of the pitch and radius of the winding, the frequency, 

 and the diameter of the wire, and is found by experiment to be sometimes as much as 

 twice the value for a straight wire. 



SMITHSONIAN PHYSICAL TABLES 



