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 banned the sale of light bulbs over 40W on 1 January 2017. The UK government announced in 2007 that incandescent bulbs would be phased out by 2011. In 2008, the Irish government announced a phase-out of the sale of any light bulbs with a luminous efficiency of less than 16 lumens per watt. Shortly afterwards, all member states of the EU agreed to a progressive phase-out of incandescent light bulbs by 2012. The initial Europe wide ban only applied to general-purpose, non-directional incandescent bulbs, so did not affect any bulbs with reflective surfaces (e.g. spotlights and halogen down lighters) or