CHAPTER II. 

 IMPEDANCE. 



Impedance. A most important consequence of the back 

 electromotive force of self-induction in a circuit is that the 

 current produced by an applied electromotive force is no 

 longer numerically equal to the voltage divided by the resis- 

 tance, and Ohm's law apparently ceases to apply. Whenever 

 an alternating voltage is applied to a circuit possessing self- 

 induction the current will be less than the value of the 



fraction volta g e . 

 resistance 



The quotient - is often called the apparent 

 current 



resistance of the circuit. This quantity is the impedance of 

 the circuit. 



Thus we have the relation 



voltage 





 impedance 



voltage 



current = 



or impedance == 



The impedance depends on two distinct properties of the 

 circuit, viz., its resistance and its self-induction, the former 

 opposing the flow of current, the latter opposing the change 

 of current. The resistance of the circuit is independent of 

 frequency of current, the shape of the conductor, or its 

 magnetic surroundings. The part of the impedance dependent 

 on self-induction varies with the frequency, and also with 

 the form and surroundings of the circuit if these affect the 

 magnetic field set up by the current. 



It has been shown above that the electromotive force to 

 be applied to a circuit in order to maintain a current of G 

 amperes in it is 



E = 



Consequently the value of the impedance of a circuit is 

 given by 



voltage C J W + (2 T n L) 2 



Impedance = - -^f_ _ l- = +/R* + (2TrnL 2 



current C 



In the triangle of electromotive force, Fig. 10, page 24, 

 each side is proportional to the current multiplied by some 



