4.  Core material around which the coil is formed—  coils are wound on either magnetic or nonmagnetic  materials. Some nonmagnetic materials include  air, copper, plastic, and glass. Magnetic materials  include nickel, iron, steel, and cobalt, which have  a permeability that provides a better path for the  magnetic lines of force and permit a stronger  magnetic field.  Since AC is in a constant state of change, the magnetic fields  within an inductor are also continuously changing and create  an inducted voltage/current. This induced voltage opposes  the applied voltage and is known as the counter EMF. This  opposition is called inductive reactance, symbolized by XL,  and is measured in ohms. This characteristic of the inductor  may also create a phase shift between voltage and current  of the circuit. The phase shift created by inductive reactance  always causes voltage to lead current. That is, the voltage of  an inductive circuit reaches its peak values before the current  reaches peak values. Additional discussions related to phase  shift are presented later in this chapter.  Inductance is the property of a circuit to oppose any change  in current and is measured in henries. Inductive reactance is  a measure of how much the countering EMF in the circuit  opposes the applied current. The inductive reactance of  a component is directly proportional to the inductance of  the component and the applied frequency to the circuit. By  increasing either the inductance or applied frequency, the  inductive reactance likewise increases and presents more  opposition to current in the circuit. This relationship is given  as XL = 2πfL Where XL = inductive reactance in ohms, L  = inductance in henries, f = frequency in cycles per second,  and π = 3.1416 In  Figure 9-18 , an AC series circuit is shown in which the  inductance is 0.146 henry and the voltage is 110 volts at a  frequency of 60 cycles per second. Inductive reactance is  determined by the following method.  In AC series circuits, inductive reactance is added like  resistances in series in a DC circuit.  [Figure 9-19]  The total  reactance in the illustrated circuit equals the sum of the  individual reactances.  X$_{L}$ = 6.28 × 60 × 0.146 X$_{L}$  = 2π × f × L X$_{LT}$ = 1 1 + 1 + 1 X$_{L1}$ X$_{L2}$ X$_{L3}$ X$_{LT}$ = 1 1 + 1 + 1  15   15   15 X$_{LT}$ = 5Ω X$_{LT}$  = 25Ω X$_{L }$= X$_{L1 }$+ X$_{L2}$ X$_{L }$ =10Ω + 15Ω 110V AC 60 Hz L = 0.146 H A AC power supply X$_{L1}$  = 10Ω X$_{L2}$  = 15Ω Figure 9-19.  Inductances in series. Figure 9-18.  AC circuit containing inductance.  The total reactance of inductors connected in parallel is  found the same way as the total resistance in a parallel  circuit.  [Figure 9-20]  Thus, the total reactance of inductances  connected in parallel, as shown, is expressed as: X$_{L}$  = 55Ω 9-13