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 Association of Dermatologists, calling for access to incandescent light bulbs for those who are medically sensitive to CFLs and other non-incandescent bulbs, and the charity Migraine Action, stating that its members still suffer adverse effects from CFLs despite protestations from the light bulb industry. In the United States, one supporter of the incandescent light bulb is the lighting designer Howard Brandston, a fellow of the Illuminating Engineering Society of North America and Honorary Fellow of the Chartered Institution of Building Services Engineers. He has attempted to raise awareness of what he believes are negative effects of the phase out through