Spirograph Bruno Abakanowicz invented the Spirograph between 1881 and 1900. It was used for calculating an area delimited by curves. Drawing toys based on gears have been around since at least 1908, when "The Marvelous Wondergraph" was advertised in the Sears catalog. An article describing how to make a Wondergraph drawing machine appeared in the "Boys Mechanic" publication in 1913. The Spirograph itself was developed by the British engineer Denys Fisher, who exhibited at the 1965 Nuremberg International Toy Fair. It was subsequently produced by his company. US distribution rights were acquired by Kenner, Inc., which introduced it to the United
Cone of curves "cone of curves" of formula_1 to be where the formula_6 are irreducible, reduced, proper curves on formula_1, and formula_18 their classes in formula_13. It is not difficult to see that formula_20 is indeed a convex cone in the sense of convex geometry. One useful application of the notion of the cone of curves is the Kleiman condition, which says that a (Cartier) divisor formula_11 on a complete variety formula_1 is ample if and only if formula_23 for any nonzero element formula_24 in formula_25, the closure of the cone of curves in the usual real topology. (In general, formula_20 need not