Model of helix or screw thread, string surface model, 1872.
© Science Museum / Science & Society Picture Library
Model of a helix or screw thread which can transform into the right line generators of a cylinder. This is one of a large set of ruled surface models made by Fabre de Lagrange, following designs introduced in the early 19th century. They illustrate surfaces which can be traced out in space by the movement of a straight line