Why do standard household light bulbs blow

00:00 Tue 10th Jul 2001 The AnswerBank

A. The thin tungsten filaments in a standard light bulb aren't designed to last forever. Although bulbs are meant to be perfectly sealed air will get in somewhere over time, the filament will get oxidised and brittle and eventually break. Bulbs that are pointed either side ways or upwards are more likely to pop as they are more likely to collapse under their own weight as they get older and more brittle. Filaments in down-warding hanging bulbs should only break if they are bumped or bashed.


Q. Older people often insist that bulbs used to last longer. Is there any truth in that

A. Yes there probably is. There have been recent well-documented cases of bulbs that have lasted for all of 70 years. Competition from overseas and the pressure to drive down costs means they just aren't made so well. In the old days bulbs would be more expensive but would, for instance, be fitted with a built-in fuse. This small strip of wire would burn out, leaving the filament intact, preventing the debris from the filament falling across the bulb terminals and causing a short circuit. Basically, what you pay for is what you get.


Q. Is there any truth in the theory that bulbs are more likely to blow in winter because our houses are cooler

A. Unlikely. If you consider that the filament heats up to 2,500 degrees C it is unlikely to make any difference if its starts of at 15 degrees C rather than 25 degrees C. Basically, we get through more light bulbs during the winter due to the simple fact that it is darker, we are in the house more and subsequently have lights on longer.


Q. Are these long-life light bulbs really such a good thing

A. They do save money, although at a cost of roughly anything between 5-15 a time you have to think long term. Essentially, what they do is use electricity far more efficiently, so a 25-watt energy efficient bulb is equivalent to a 100-watt ordinary bulb and as a result uses about as quarter as much electricity. They also last about 10-12 times longer so you save on the equivalent number of replacements. If you can't afford to change all the bulbs in your house to energy-saving ones start replacing the most used bulbs (in the hall or kitchen, for instance) one at a time.


Q. What developments can we expect in the future

A. There is a lot of excitement about the potential of LEDs (light-emitting diodes). Without getting too technical, these have been with us since the 1960's and what they basically do is turn electricity directly into light, without the need to heat it up, i.e without the filament.

Until recently LEDs came in red orange and green but until recently not blue, needed to produce white light which is made up of a mixture of colours. This has now been done and LEDs are now being used on an industrial scale, although it could be a few years yet before they are developed to be cost effective in household use. When they do it is estimated they will use 85 per cent less electricity and last 16 times longer than the light bulb.


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By Tom Gard


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