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From Sam Edge, Ringwood, Hampshire, UK

You say that a hygroelectric generator based on graphene produces 1.5 volts and that this is equivalent to an AA or AAA battery (1 December 2018, p 20). But I want to know what current it delivers, and so its power in watts.

The editor writes:
• The experimental device – and an AA cell . What was interesting was that it was possible at all.