A coil is nothing more than copper wire wound in a spiral.
 This symbol is used to indicate a coil in a circuit diagram.
 Inductance value is designated in units called the Henry(H). The more wire the coil contains, the stronger its characteristics become. The inductance value can become quite large. If a coil is wound around an iron rod, or ferrite core (strengthened with iron powder), the inductance of the coil will be greatly increased. Coils used in typical electric circuits varely widely in values, ranging from a few micro-henry (H) to many henry (H).
 
 Coils are sometimes called "inductors." Inductance is the measure of the strength of a coil. Capacitors have capacitance, resistors have resistance, and Inductors (coils) have inductance. When alternating current flows through a coil, the magnetic flux that occurs in the coil changes with the current. When a second coil is put close to the first coil (with the changing flux), alternating voltage is caused to flow in the second coil by an effect known as "mutual induction." Mutual inductance (inductance) is measured in units of the Henry. The changing magnetic flux in a coil affects itself as well as other coils. This is called self induction, the degree of this self induction is called Self Inductance. Self inductance is a measure of a coil's ability to establish an induced voltage as a result of a change in its current. Self inductance is commonly referred to as simply "inductance," and is symbolized by "L". The unit of inductance is the Henry (H).
 
 The definition of "Henry" is "When a current of 1 ampere flows through a given coil in 1 second such that 1 volt is induced to flow in a second coil, the mutual inductance between the coils is said to be 1 Henry." The definition of self inductance is the same, except that the 1 volt is induced in the first coil; there is no second coil.  

 
  Characteristic of coils
 When wire is coiled, it takes on various characteristics that are different from straight wire. Below I will explain some of the characteristics coils that I know.
 
  Current Stabilization Characteristic
 When current begins to flow in the coil, the coil resists the flow. When current decreases, the coil makes current continue to flow (briefly) at the previous rate.
 This is called "Lenz's law".
 'The direction of induced current in a coil is such that is opposes the change in the magnetic field that procduced it.'
 
  Mutual induction
 As I wrote above, electric power can be transfered between two coils by mutual induction.
 
 The transformer utilizes this characteristic.
 The input coil that gives the electric power is called the primary side, while the output coil that takes out the electric power is called the secondary side.
 The output voltage is determined by the ratio of turns of wire between the primary coil and secondary coil.
 
 Some transformers have a tap (or several) on the secondary coil to provide multiple voltage levels.
 
  Electromagnet
 When current flows through a conductor, a magnetic field is created. This field is much stronger in a coil. An electromagnet is just like a regular magnet. It attracts iron, nickel, and some other metals.
 
 Relays utilize this characteristic.
 When the current flows to the coil of relay, the magnetic field attracts a steel plate, and the switch that is attached to the steel plate goes ON. And the doorbell chime also utilizes electromagnets.
 
  Resonance
 When a coil and a capacitor are combined, the resulting circuit has special characteristics. The impedance (resistance to current flow) of the circuit changes with the frequency of the voltage. Current will flow easily at a given frequency, but has difficulty flowing at another frequency.
 The tunning circut that select a particular radio station utilizes this characteristic.
 Explaning resonance in more detail is very difficult. If you want to know more detail, please read further in a book about electronics.
 

 
 
 
 

 
 
 
  High Frequency Coils
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