Before the 1970s, for anything but basic arithmetic, a pen and paper was needed, and complex calculations required a slide rule or logarithm tables.Electronic calculators were in use in the 1960s, but they were based on valve or transistor technology, and so they were both bulky and expensive machines.But with the development of the microchip on the back of the space race, the rapid process of miniaturisation began.By the early 1970s, affordable pocket calculators came to the shops and enabled everyone to have instant answers to any mathematical question.A global market worth hundreds of millions of dollars emerged and the large electronics companies in Japan and the US were joined in a battle to produce ever smaller and cheaper machines.In the UK, companies such as Sinclair also wanted to take a share of this expanding market. Andrew Morten tells the tale of the pocket-sized revolution of the innovative pocket calculator.