362 ELEMENTS OF ELECTRICAL ENGINEERING. 



so that 



It is to be particularly noted that the inductance of a coil of 

 wire wound on an iron core varies with degree of saturation of 

 the iron, inasmuch as the permeability of the iron is variable. 



Case III. The inductance of a transmission line is given by 

 the equation 



L = 0.0017 x Iog 10 -g x / (8) 



in which L is the inductance of the line in henrys, / is the 

 length of the line (actual length of row of poles) in miles, d is 

 the distance apart of the two wires center to center, and R is the 

 radius of the wires, d and R being both expressed in the same 

 units. 



8. Electric charge. A current of water through a pipe is a 

 transfer of water along the pipe, and if q be the amount of water 

 which during / seconds flows past a given point in the pipe, the 

 quotient q\t is the rate of flow of water through the pipe, and 

 this rate of flow may be spoken of as the strength, i t of the water 

 current. Suppose the strength i of the water current to be given 

 (rate of flow of water) then the amount of water flowing past a 

 given point of the pipe in t seconds is given by the equation 



q = it 



Similarly, an electric current in a wire may be looked upon as 

 a transfer of electricity along the wire, and the quantity q of 

 electricity which flows past a point on the wire during t seconds 

 may be defined as the product of the strength of the current and 

 the time. That is 



9 = * (9) 



If the strength of the electric current is variable, equation (9) 

 must be written in the form 



^q i'^t (10) 



