'Mobile electricity' - that is, electricity available on the move - is the most valuable form of electrical power. This is why it is worthwhile equipping notebooks and laptops with expensive lithium-ion batteries that need to be recharged at regular intervals by plugging them into power sockets. The iPad, which looks set to spur the market for electronic reader devices, remains uninteresting without its batteries. However, solar cells and hand cranks are already able to generate standalone power for mobile devices. Will these devices be able to cope entirely on their own without power sockets someday?