I have a ceiling light in my office that went out. The tubes were the same length as another light in my basement for which I had some spare tubes, but these have a smaller diameter than the burnt-out ones. The prongs matched so I put them in. The light doesn't really work now. Both tubes glow faintly, then sputter out blasts of light randomly. A few times the light has actually shown brightly and steadily. I said to myself at these times, "Oh fine, I will just leave the light on all the time now." Then, upon leaving the room I have turned off the light each time. Halfway up the stairs I have said dammit but that doesn't help much.