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You'd be forgiven for imagining that one of the benefits of e-books is that, unlike heavy hardbacks, a digital best-seller is entirely weightless。 However, this is not quite correct, according to a computer scientist - a Kindle full of downloads weighs more than an empty one。 But even the keenest readers are unlikely to suffer too much of a burden, as a full library of e-books weighs no more than a single molecule of DNA。 According to John Kubiatowicz, from the University of California, Berkeley, the difference between an empty e-reader and a full one is just one attogram, or 10-18 of a gram。 This weight is so tiny as to be "effectively unmeasurable" by any scales that currently exist, he says。 The difference in weight is far less than the difference produced by charging the battery, or wiping dust off the screen。 And electronic devices tend to grow heavier in sunlight and in warm weather, a change which massively outstrips the effect of downloading more data。 By comparison, 3,500 paperbacks - equivalent to the capacity of the Kindle - would weight around two tonnes。 Prof Kubiatowicz's findings are based on the fact that, while downloading an e-book does not change the number of electrons in an e-reader, those electrons holding data have a higher level of energy。 The relationship between energy and mass - famously summarised by Einstein as E=mc2 - means that those with a higher energy also have a higher mass。 | 
