is elongated to permit the key to move longitudinally with the adjustment mandrel relative to the adjustment sleeve as seen in fig2 a window 78 is milled into adjustment sleeve 40 so that the extent of longitudinal movement of adjustment mandrel 38 relative to adjustment sleeve 40 and consequently housing 14 can be monitored an index mark 77 on the mandrel is visible through the window 78 adjustment sleeve 40 also includes indicia in the form of marks 80 82 and 84 to identify particular trigger load settings alignment of mark 77 with one of marks 80 82 84 is an indication of a high medium or low trigger load setting the extent of adjustment of adjustment mandrel 38 is preferably on the order of one half to one inch from a nominal setting referring to fig3 a 3d inclusive there is shown an exemplary embodiment of a jar 100 adapted to be attached to a wireline and inserted into a well borehole not shown the jar 100 has a mandrel 112 disposed within tubular housing 114 the mandrel 112 is axially movable with respect to housing 114 a bore 116 extends the length of mandrel 112 an elongated conductor rod 120 electrically insulated from the mandrel 112 and the housing 114 by an insulating sleeve 122 extends through bore 116 to bulkhead 124 the upper end of mandrel 112 is provided with threads.