Human Powered Devices
Pictured is a collection of human powered devices. A push along lawnmower, a bicycle (with dynamo front light), a wind-up alarm clock, a wind up pocket watch, a wind up radio and a wind up torch.
In the alarm clock and the pocket watch energy is stored in a spring which, as it unwinds, powers the mechanism.
Usefulness = (Functionality x Availability) – Burden