Incandescent light bulb Small variations in resistivity along the filament cause "hot spots" to form at points of higher resistivity; a variation of diameter of only 1% will cause a 25% reduction in service life. These hot spots evaporate faster than the rest of the filament, which increases the resistance at that point—this creates a positive feedback that ends in the familiar tiny gap in an otherwise healthy-looking filament. Irving Langmuir found that an inert gas, instead of vacuum, would retard evaporation. General service incandescent light bulbs over about 25 watts in rating are now filled with a mixture of mostly argon and
Phase-out of incandescent light bulbs special purpose bulbs including those used in devices such as household appliances, traffic lights, infrared lamps and automotive lighting. The sale of the most inefficient bulbs was phased out. The first types to go were non-clear (frosted) bulbs, which were taken off the market in September 2009. Also from September 2009 clear bulbs over 100 W were made of more efficient types. This limit was moved down to lower wattages, and the efficiency levels raised by the end of 2012. In practice, some manufacturers and retailers have found a loophole in the new rules so that some incandescents are still