Spark

i know this was discussed not long ago but could someone (racer or kim) please explain to me again why the plug must be removed from the equation when testing to see if you have good spark(3/8 gap,bright and snappy) and why you cant or should not ground your plug to the block to check spark. my buddy is having problems starting his motor and said he checked the spark(using the plug) and its good and i told him thats not right way to test and he dont beleive me and neither one of us understand why you cant use plug but i do know you cant. thanks alot!!

The manufacturers ( at great expense ) have equipped outboard motors with ignition systems that can produce up to 50,000 volts.------------THERE MUST BE A REASON.------------With a plug laying on the block you see a spark.-------That means nothing really.---------------It takes more voltage to jump a plug gap when the plug is in the cylinder.----------The air in the gap is now compressed to say 100 PSI instead of 14.6 PSI which is atmospheric pressure.----------One of the most common mis- understood concepts ??