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 have had exchange programs to replace incandescent bulbs with energy efficient versions. Ghana banned incandescent bulbs in 2008 and Senegal has banned the manufacture and import. Egypt is reported to have a partial ban on incandescent light bulbs. Kenya has a programme to exchange a number of light bulbs with more efficient types. Morocco has a programme to exchange a number of light bulbs with more efficient types. Namibia has an exchange program which distributed 1 million energy saving bulbs in 2016. Rwanda has an exchange program, distributing CFL bulbs. South Africa phased out incandescent light bulbs in 2016. Tanzania