Jesse Ramsden: Circular Dividing Engines
This is a sub-section of Jesse Ramsden
Precisely divided circular scales were of vital importance for instruments used in navigation, surveying and astronomy. In the late 18th century English instrument-maker Jesse Ramsden produced several highly-regarded machines for producing graduated circular scales.
Ramsden's machine won him an award from the Board of Longitude. As a condition of the reward, Ramsden was required to provide full details of its design and construction, and to teach others to construct and use the machines. Ramsden's account provides a valuable insight into his remarkable appreciation of the principles of precision engineering, and his ability to produce the required result with the simple equipment available at the time. Even though Ramsden described the processes in great detail, the number of people having the skill, expertise and patience to replicate his achievement were few indeed.
Ramsden's description is greatly augmented by a commentary produced by Bill Morris, available online here. See also background here.
Ramsden's instrument had a mahogany frame with three legs. The frame supported a heavy wheel, 45 inches diameter, cast in bell metal. Attached to its upper face was a brass ring, cut with 2160 gear teeth. These teeth engaged with a worm on one side of the machine. Turning this worm 6 times rotated the wheel 1 degree. The worm's spindle had a micrometer dial with 60 divisions, each representing 10 seconds rotation of the wheel. The workpiece to be divided was clamped to the arms of the bronze wheel, beneath the graving stylus on its traversing carriage.
The 45" wheel had a tapered bearing at its centre, and low-friction support was provided by three conical wheels.
Maurice Daumas states that Ramsden's first machine was built in 1766, but it did not satisfy him.[1]). This is confirmed by Morris[2], who states that in about 1766 Ramsden completed his first circular dividing engine, which was superior to all others then existing, but with errors of up to three arc seconds, it did not meet his exacting standards. His second version was completed by mid-1774, and the claimed error was less than 1 second. Daumas states that Ramsden sold the first machine to Bochard de Saron 'after having ensured that its condition had deteriorated so that it was unusable. It appears that this machine was refurbished and modified by E. Lenoir to divide a circle into 400 parts in connection with the construction of repeating circles for the Commission du Metre. This machine is currently (2019) on display in the Musée des Arts et Metiers. See photos P1-P5.
The National Museum of American History have an example made by Ramsden in 1775. See here. It had been purchased from Ramsden's successors and went to the USA in 1850. The Philadelphia firm of Knox and Shain (Joseph Knox and Charles J. Shain) purchased it for their own use.
The Science Museum in London have an example made by John Troughton in 1778. Description and illustrations here. Note: Daumas states subsequently Troughton's workshop made a number of other dividing engines, the screws being produced by Samuel Rhee.[3].
A circular dividing engine made by Samuel Rhee on the basis of Ramsden's machine, and purchased by Jean Antoine Chaptal (1756-1832), is in the possession of the Musée des Arts et Metiers, but is not on display. Basic information here. Photographs here and here.
See Also
Sources of Information
- ↑ 'Scientific Instruments of the 17th & 18th Centuries and their Makers' by Maurice Daumas, translated by Dr Mary Holbrook, Portman Books, 1972
- ↑ [1] 'A Commentary of Jesse Ramsden's Circular Dividing Engine' by Bill Morris, 2010, p.4
- ↑ 'Scientific Instruments of the 17th & 18th Centuries and their Makers' by Maurice Daumas, translated by Dr Mary Holbrook, Portman Books, 1972, p.202