INDUCTION COIL

INDUCTION COIL is a coil of discharging type, a kind of
transformer used to output high-voltage pulses starting from
a low-voltage direct-current power supply. The direct current
crossing the primary winding is cut off repeatedly by a vibrating
contact called switch, to provoke the necessary variation of
flux that induces the desired voltage in the secondary winding.
The term ‘induction coil’ is also used to indicate a coil crossed
by a high-frequency alternating current that heats the objects
placed inside it as it occurs in induction furnaces.
This coil consists of two solenoids of insulated copper wire
wound around a unique iron core. One of these solenoids, called
primary winding, consists of tens or hundreds of wire turns,
whereas the other solenoid, called secondary winding, consists
of several thousands of turns of thin wire. When crossing the
primary winding, electric current creates a magnetic field and
the secondary winding is coupled magnetically through the
iron core. The primary winding acts as field magnet and stores
energy in the corresponding magnetic field. When electric
current is cut off suddenly, the magnetic field is reduced rapidly
and this provokes a high-voltage pulse through the secondary
winding for electromagnetic induction.
Thanks to the high number of turns of the secondary winding
this output pulse has a voltage of many thousands of volts. This
voltage is sufficient to produce a spark or electric discharge
across the air separating the terminals of the secondary
winding.
The size of an induction coil is often indicated according to the
length of the spark that it can produce.