(Spark-) Induction Coils

Induction coils fascinated me, from the very beginning of my interest in electricity. They're really ancient devices (like an "older brother" of Tesla coils) , invented and developed around 1850 by famous people from J.Henry to H.Ruhmkorff and others (see also Lyonel Baum's site ). Very widespread contemporary application is in automotive ignition coils, relevant to tesla coilers as power supplies for tiny TC's, but more so as HV elements in the recent development of triggered spark gaps. The analysis of induction coils reveals a close proximity to the one on Tesla coils. They're NOT just simpler than Tesla coils, at least at the beginners level (speak: lumped parameter models). The main differences are:

1.) Starting energy is stored as Li2/2, instead of CU 2/2 in TC's (at least in conventional "interrupter"-mode).
2.) The primary and secondary LCR circuits are NOT tuned to the same resonant frequency.
3.) The coupling coefficient k of induction coils is much higher (>0.9 etc.) than the one of TC's (0.1..0.2 typ.). 
4.) Induction coils have an iron core vs. the "air core" of Tesla coils. 

Relevant common properties of TC's and induction coils:

1.) Both should be treated as inductively coupled LCR circuits; not exclusively as  "AC transformers".
2.) Certain coupling coefficients may be "magic" or "optimal", in order to generate high secondary voltage.
3.) The behavior can be accepted as dominantly linear, because of the open core of the induction coil. This
     statement might be challenged, by those beeing able, to drive the open core to saturation ! :-).
     (however: the iron loss of the induction coil must be considered for the damping behavior). 

The really weak element of the old induction coils was  the "interrupter", being it a Wagner-Hammer, a Mercury Turbine Interrupter or a Wehnelt Electrolytic Breaker. Modern power electronic devices offer an alternative: if designed and tested, based on a good simulation model of the induction coil itself, they will probably outperform the ancient interruptors, and, - may be -, get the induction coil again into consideration for certain applications, as was the case before, i.e. with the old X-ray machines.

Sparks from
Kurt's Inductor

Andi's Inductor

Process of Construction 34 Tests with
Kurt's Inductor

Induction Coil Theory & Analysis Pspice Simulation