article: First, he used a prehistoric technology: he lit a wood fire. But Prof Nordhaus also had a piece of hi-tech equipment with him - a Minolta light meter. He burned 20lb (9kg) of wood, kept track of how long it burned for and carefully recorded the dim, flickering firelight with his meter. Next, he bought a Roman oil lamp, fitted it with a wick, and filled it with cold-pressed sesame oil. He lit the lamp and watched the oil burn down, again using the light meter to measure its soft, even glow. Bill Nordhaus's open wood fire had burned for just three hours on 9kg of wood. But a mere eggcup of oil burned all day, and more brightly and controllably. Why did he do this? He wanted to understand the economic significance of the light bulb. But Prof Nordhaus also wanted to illuminate a difficult issue for economists: how to keep track of inflation, the changing cost of goods and services. 50 Things That Made the Modern Economy highlights the inventions, ideas and innovations that helped create the economic world. It is broadcast on the BBC World Service. You can find more information about the programme's sources and listen online or subscribe to the programme podcast. To see why this is difficult, consider the price of travelling from - say - Lisbon in Portugal to Luanda in Angola. When that journey was first made, by Portuguese explorers, it would have been an epic expedition, possibly taking months. Later, by steam ship, it would have taken a few days; then, by plane, a few hours. An economic historian could start by tracking the price of passage on the ship, but once an air route has opened up, which price do you look at? Maybe you simply switch to the airline ticket price once more people start flying than sailing. But flying is a different service - faster, more convenient. If more travellers are willing to pay twice as much to fly, it hardly makes sense for inflation statistics to record that the cost of the journey has suddenly doubled. It was to raise this question over the way we measure inflation that Bill Nordhaus started fooling around with wood fires, oil lamps and light meters. He wanted to unbundle the cost of a single quality that humans have cared deeply about since time immemorial, using the state-of-the-art technology of different ages: illumination. Light is measured in lumens, or lumen-hours. A candle gives off 13 lumens while it burns. A typical modern light bulb is almost 100 times brighter than that. Imagine gathering and chopping wood 10 hours a day for six days. Those 60 hours of work would produce 1,000 lumen hours of light. That is the equivalent of one modern light bulb shining for just 54 minutes, although what you would actually get is many more hours of dim, flickering light instead. Of course, light is not the only reason to burn fires: they also help keep you warm, cook your food and scare off wild animals. If you just needed
summary: Back in the mid-1990s, an economist called William Nordhaus conducted a series of simple experiments with light.