Good analysis George, (fun for us sparkies at least) A standard old tractor can type ignition coil, like many other electrical devices having wire (and coils have a bunch) change resistance as their temperature changes. Of course, in these low RPM few (1,2,4, maybe 6 Ollies) cyliner old tractor engines theres plenty sufficient dwell time to saturate the coil UNLIKE an 8 cylinder engine running 8000 RPM all through a single set of points AHHH the Mallory Dual Point distributors like when we were kids lol
As you note the "duty cycle" changes versus RPM and as time passes shes warmed up thereby increasing the coils resistance. Most old tractors didnt, however, use any stand alone discrete thermistor type (temp compensating) of ballast device like some Fords used and new gen Deere started using.
Still, when you walk into NAPA or Central Tractor or Tractor Supply and ask for a coil for an old tractor with points, theyre gonna ask if you want a "6 volt" or a "12 volt" and will NOT say "theyre all the same" OR they have a Universal model suitable for use with EITHER 6 or 12 volts across its terminals (NOTE thats NOT to say you cant use a 6 volt coil on a 12 volt tractor with an external series voltage dropping (12 down to 6) ballast) . They will be labeled "6 volts" or "12 volts" or "12 volts does NOT require external ballast" or "12 volts for use with ballast" (which is in reality a 6 volt coil as the ballast drops 6 volts leaving 6 on the coil) Also, if you use the wrong coil theres problems, if you apply 12 volts to a coil designed for 6 volts across its terminals, she overheats, while if you apply only 6 to one designed for 12 volts across its terminals, the spark is weak...
AS YOU WELL NOTED, YOU NEED THE CORRECT COIL AND CORRECT BALLAST AND/OR COMBINATION
In keeping with standards of use and the common terminology used in the trade, I would define (like the Industry does) what the stores call and what are clearly labeled 12 VOLT COILS as ones designed for application of a NOMINAL 12 volts across their terminals which can still produce a spark (albeit weaker) if the battery voltage is reduced (as during cranking) and also produce a spark when the battEry voltage is raised due to charging, yet still function and not overheat. Such are labled and called in the trade 12 VOLT COILS, NOT 6 OR 12 volt coils LOL.
Many (not all) 12 volt coils (per the industry definition of and labeled as a 12 volt) have a LV primary winding resistance of over 2 ohms up to near 4, with an average of around say 3 ohms. That way the points carry and switch around 4 amps as if they switch too much current they burn up prematurely. Many (not all) coils labeled 6 volts (and as defined above) have a LV primary winding resistance of over 1 up to around 2 ohms, with a typical average of 1.5 ohms WHICH AGAIN ON A 6 VOLT TRACTOR allows the points to carry and switch 4 amps so they dont burn up prematurely.
Of course you can use a 6 volt coil on EITHER a 6 or 12 volt tractor (like so many tractors did) so long as if on a 12 volt you add the external voltage dropping ballast WELL DUH otherwise the coil would overheat and the points (drawing too much current) will burn up quickly
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