Demenÿ in 1893, to achieve intermittent film transport. Geneva wheels having the form of the driven wheel were also used in mechanical watches, but not in a drive, rather to limit the tension of the spring, such that it would operate only in the range where its elastic force is nearly linear. If one of the slots of the driven wheel is occluded, the number of rotations the drive wheel can make is limited. In watches, the "drive" wheel is the one that winds up the spring, and the Geneva wheel with four or five spokes and one closed slot prevents overwinding (and also complete unwinding) of the spring. This so-called Geneva stop or "Geneva stop work" was the invention of 17th or 18th century watchmakers. Other applications of the Geneva drive include the pen change mechanism in plotters, automated sampling devices, banknote counting machines, and many forms of indexable equipment used in manufacturing (such as the tool changers in CNC machines; the turrets of turret lathes, screw machines, and turret drills; some kinds of indexing heads and rotary tables; and so on). The Iron Ring Clock uses a Geneva mechanism to provide intermittent motion to one of its rings. A Geneva drive was used to change filters in the Dawn mission framing camera used to image the asteroid 4 Vesta in 2011. It was selected to ensure that should the mechanism fail at least one filter would be usable. Internal version An internal Geneva drive is a variant on the design. The axis of the drive wheel of the internal drive can have a bearing only on one side. The angle by which the drive wheel has to rotate to effect one step rotation of the driven wheel is always smaller than 180° in an external Geneva drive and always greater than 180° in an internal one, where the switch time is therefore greater than the time the driven wheel stands still. The external form is the more common, as it can be built smaller and can withstand higher mechanical stresses. Spherical version Another variant is the spherical Geneva drive. Kinematics The figure shows the motion curves for an external four-slot Geneva drive, in arbitrary units. A discontinuity appears in the acceleration when the drive pin enters and leaves the slot, occurring at the instant the rigid bearing surfaces make contact or separate. This generates an "infinite" peak of