The coils are step-up transformers, which means they amplify, or "step-up", the voltage. If the energy does not have somewhere to go, to return to ground, it will arc across the wire windings within the coil, burning through the insulation, destroying the coil, rendering it into scrap metal (copper).
To check for spark, Luke (?) stated in another thread, earlier today, just
use an inductive timing light, which has a clip that goes around a spark plug wire, picking up an induced voltage from the high voltage current going through the wire.
DO NOT ATTEMPT TO HOLD THE PLUG IN YOUR HAND TO GROUND IT TO THE ENGINE BLOCK WHILE FIRING!!!WARNING! 45kV is 45,000 volts! There is very little current, but
that level of voltage can cause a heart arrhythmia (abnormal heart beat), which may not produce symptoms immediately, but which could lead to your heart stopping, suddenly, with little, to no warning! On all modern car ignition systems, the voltage is around this level, and getting shocked by it, requires a mandatory trip to the ER to get your heart checked for a normal beat...
The old, 70's vintage coils, produce voltages probably less than 15kV -- still painful, and still capable of causing a heart arrhythmia, but not nearly as dangerous as the modern, 45kV units. One method of improving the power output of the OEM coils, is to use a 12V relay, to switch the coils on (requires a solid-state, high-speed relay) connected directly to the battery. This approach will give the coils full battery voltage, and current (there are numerous resistive losses in the wiring between the coils' input tabs, and the battery terminals, typically).
If you go with these coils, I would strongly recommend NKG Iridium Spark Plugs ($8/plug), which are also resistive. If the OEM coils are still working, try the Iridium Plugs, alone, and first: inexpensive compared to new coils, and they require less energy to throw a spark -- they fire much easier, much more reliably, and you may not need anything more than these. Cheers!
1979 CB750K (sold, 2012, but not forgotten)
1983 Kawasaki 440 LTD Belt Drive (sold, 2011)
1993 Kawasaki Voyager XII