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Climbing/caving equipment. A brand of ascender but now often used as a generic term to describe all ascenders.

The Jumar was developed by a Swiss, Julias Marte in 1958 to climb down to eagle nests to band baby eagles, but earlier versions of the mechanism were developed by Henri Brunot in 1947 for scaling the inside of a mountain, the Reseau de la Dent de Drolles in France. It wasnt until the late 60's that cavers at large, adopted the technology to get out of large holes that were discovered in Mexico.

Prior to this point climbers and cavers would use prusik knots to ascend ropes.

Various climbing and caving equipment companies including the industrial design wizards Petzl, have come up with their own take on the ascender but all use the same simple design with a spring loaded cam jamming the rope against a shallow groove with a handle to hang onto or a hole to attach a carabiner to.

unengaged - cam has rotated out of the way as rope is moving through groove from top to bottom (or jumar is moving up the rope).Distance from pivot to the edge of cam contacting rope decreases till pressure on rope is zero

                  rope
                 |\\\\\\\|
              II |\\\\\\\|
              II |\\\\\\\|
              II |\\\\\\\|       ____
              II |\\\\\\\|    ___|///|
              II |\\\\\\\| ___|////O//| pivot
              II |\\\\\\\| |//////////|
       groove II |\\\\\\\|  |/////////|
              II |\\\\\\\|   |//cam///|
              II |\\\\\\\|    |///////|
              II |\\\\\\\|      |/////|
              II |\\\\\\\|        |//|
              II |\\\\\\\|                         
                 |\\\\\\\|

engaged - cam has rotated into the rope with the action of its spring, as the user tries to move rope from bottom to top (or trying to move jumar down the rope). Distance from pivot to the edge of cam contacting rope increases till pressure on rope is a maximum

                  
                 rope
                 |\\\\\\\|
              II |\\\\\\\|
              II |\\\\\\\|
              II |\\\\\\\|      _____
              II |\\\\\\|______|////|
              II |\\\\\||////////O//|  pivot
              II |\\\\\||//////////|   
       groove II |\\\\\\||/////////|    
              II |\\\\\\\||///cam//|   
              II |\\\\\\\||///////|   
              II |\\\\\\\|  |/////|  
              II |\\\\\\\|    |//|   
              II |\\\\\\\|                 
              II |\\\\\\\|
                 |\\\\\\\|

some data from http://www.nps.gov/cave/pdfdocs/c&c4.pdf
http://www.caves.org/section/hss/pu93v71.htm

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