Something I always wondered about was a long time ago back in the early 1960s I had several 15 and 20 watt fluorescent bulbs that said on the bulbs "for AC or DC service".

What does that mean?
Is there a DC ballast of some sort?
Just what type of fixture would you use such a bulb in?
Would one end of the bulb be positive and the other negative?
They did work in a regular AC fixture.
But I never seen a DC fixture before.

Just how does this work?
 

The fluorescent work at both DC and AC current, with DC you have to adress somehow the electrolysis.
For that the lamp need somethingto limit the current. And the easiest component to limit the current is just a resistor. And the resistor work on both DC and AC, so if the fixture uses resistor as it's ballast, it will work on both. As the resistors

The disadvantage of a resistor are it's losses, with a F20T12 on a 120V, the resistor losses are about 25..30W, so the complete fixture consumes about 45..50W. So on AC, when you can implementthe series impedance without the large losses by an inductor, the inductors became the standard form of ballasts. And because the inductor limit the current only on AC, these fixtures were limited to just an AC.

The fluorescent work at both DC and AC current, with DC you have to adress somehow the electrolysis.
For that the lamp need somethingto limit the current. And the easiest component to limit the current is just a resistor. And the resistor work on both DC and AC, so if the fixture uses resistor as it's ballast, it will work on both. As the resistors

The disadvantage of a resistor are it's losses, with a F20T12 on a 120V, the resistor losses are about 25..30W, so the complete fixture consumes about 45..50W. So on AC, when you can implementthe series impedance without the large losses by an inductor, the inductors became the standard form of ballasts. And because the inductor limit the current only on AC, these fixtures were limited to just an AC.

Thanks Medved,
I know there were many places where DC was still used for lighting even after AC was the standard. May be that was where the bulbs were used. I just never seen a DC fixture before. May be someone has a picture of one.

There were plenty of fixtures using special incandescent lamps as resistive ballasts - the main reason was to lower the cost of the (at the time) novelty fluorescent systems. So as the resistor (so the incandescent) principally works on both AC and DC, I won't be surprised someone putting the "DC" in the official rating of the fixture, even when the DC was not anymore in so wide use at times the fluorescents arrived. It was used mainly for marketing reason: To show the resistive systems have at least some advantages over the inductive ballasts other than the cost. When the related patents expired, many makers stepped into the magnetic ballast business, so the magnetic cost fell dramatically down, making the inefficient resistive ballasts practically not sellable... But of course, that "advantage" (the promoted ability to run even on DC) wasn't able to save the resistive/incandescent ballast systems, there were not so many customers really needing such feature...