A device powered by a potato which enables stepping between parallel Earths in The Long Earth. Plans of which were posted on the internet, which enabled many people- mostly children- to build their own Steppers.
It appears in the form of a small box, with various wires inside it, attached to the power source, the said potato. On top of it there is a switch, which can be pointed to 'East', 'Off', or 'West'. It is unknown how it works.
It was invented by Willis Linsay, who has been described variously as a genius, a psychopath and a very dangerous man to know.
An animation of how the stepper is put together can be seen here .