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Once too precious to use, now too cheap to notice - the significance of the lightbulb is profound. Imagine a hard week's work gathering and chopping wood, ten hours a day for six days. Those 60 hours of work would produce light equivalent to one modern bulb shining for just 54 minutes. The invention of tallow candles made life a little easier. If you spent a whole week making them - unpleasant work - you would have enough to burn one for two hours and twenty minutes every evening for a year. Every subsequent technology was expensive, and labour-intensive. And none produced a strong, steady light. Then, as Tim Harford explains, Thomas Edison came along with the lightbulb and changed everything, turning our economy into one where we can work whenever we want to.