An induction coil, often referred to as a Ruhkorff coil, is the oldest and first commercially produced voltage transformer. Today electrical transformers find ubiquity in most electronic devices satisfying a variety of roles with regards to altering voltages. Modern automotive ignition coils can trace their development directly back to the induction coil of the 1800's.
In the case of Mr. Ruhmkorff’s coil, low DC voltages of electricity are multiplied to very high DC low current voltages which are analogous to static discharges. The archetypal induction coil will multiply 6-12 volts to 25,000-120,000 volts with ease. The largest induction coil ever constructed, had the ability to produce 40+ inch sparks @ 1,000,000+ volts!
How the Induction Coil Works
When electricity is passed through the primary coil, a magnetic field is created and amplified due to the iron core. This electromagnetic field "induces" a current in the secondary coil, hence the name, induction coil. For successful induction in the secondary, there has to be continuous power switching in the primary source of the current must occur. The result of the rapid switching is a high voltage pulse passed on across the secondary coil a process referred to as electromagnetic induction.
In mechanical induction coils the interrupt mechanism, nothing more than a switch on a flat spring, is attracted to the magnetic field induced by the iron core. Once contact is broken the core looses it's magnetic field and the trembler springs back to the on position repeating the process. Electronic induction coils, most commonly automotive ignition coils, switch on and off due to the use of transistors.