Coradi planimeter and map, 1886.
© Science Museum / Science & Society Picture Library
A planimeter made by G Coradi, Zurich, Switzerland. A planimeter mechanically measures the area bound by a closed curve. They are used to measure maps, plans and the area or volume of an engineering component. In the instance of an area on a map it is achieved by fixing the anchor end of two hinged rods into the drawing board whilst the end of the other rod traces the boundary line of the area to be measured.