Is it true that T8 fluorescent bulbs have more light output than a T12 of the same wattage? Also I was told that if one T8 bulb burns out then the others will stay on as compared to a T12 where if one bulb burns out then they all go out. Wouldnt this depend on the type of ballast that you are using?
When T8s fail, usually only that bulb will go bad, if you see 2 T8s out, its almost always a bad ballast.
T12s on the other hand can have a single lamp go out or both lamps. I think it depends on how the lamp fails internally. Also, if you remove a T12 from a 2 lamp ballast, it will shut off the other light, a T8 ballast will power a single lamp with no problem.
I think you are correct that this is more to do with electronic vs magnetic ballasts.
T8 4 bulb 2 by 4 troffer luminaire becomes a lot lighter than the T12 so light if installed in suspended ceiling and it is not fasten properly you could push the whole luminaire out of the ceiling grid. The ballast only requires 1 not 2 and they weigh one fourth of ther T12 ballast. Never checked lumens but heard from customers it is alot brighter.
The "eighths of an inch diameter" thing goes back to the early days of lamps and radio tubes, and lives on to this day. Even LEDs are commonly specified this way (T1-3/4 is a common package).
The number of eighths in the diameter is also the basis of the NEMA motor frame designations, AFAIK. For example, a 56 frame motor was originally 56/8ths " in diameter, or 7". As motors have improved, they have gotten a bit smaller, but the numbers still provide a rough idea of size.