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 to one-third the electric power, and may last eight to fifteen times longer. Newer phosphor formulations have improved the perceived color, with "soft white" CFLs judged subjectively similar to standard incandescent lamps. Objections more specifically relating to compact fluorescent light bulbs include the different quality of light produced by phosphor-based lamps compared to incandescent lamps and that compact fluorescent light bulbs contain small amounts of mercury, a potent neurotoxin, which is especially dangerous to children and pregnant women, and made more serious by the confined area mercury would be dispersed in the event of a breakage indoors. Environmental concerns about