Why can compact fluorescent bulbs be used upside down? What is the problem they have with dimmers?
They *can* be used upside down. The mains is alternating current, which means the voltage starts from zero, rises to maximum positive, drops to zero, falls to maximum negative and returns to zero. This happens several times a second — usually 50 or 60. It will say on your electricity meter. A dimmer switch works with an electronic switching device called a triac; which stays switched off at the beginning of the cycle. The longer it stays off, the less power is supplied to the bulb, so it glows more dimly. Fluorescent lights already have a different kind of limiting device — either a simple ballast coil, or a more sophisticated electronic circuit — which is necessary because of the way they work; but either way, it’s based on a different phenomenon in electronics, and you can’t use the two methods together.