of circular cross section and sleeve 19 of annular cross section disposed around rod 18 rod 18 and sleeve 19 are dimensioned so that they can slip relative to one another rod 18 is advantageously 1 4 inch diameter sleeve 5 16 inch outer diameter 0 028 inch wall thickness bottom end 21 of rod 18 is affixed to bottom end 22 of sleeve 19 by bottom fitting 23 top fitting 24 is affixed to top 25 of sleeve 18 and provides spring bearing 26 and signal object 27 rod and sleeve assembly 17 is positioned with its top end 20 in housing 15 and its lower end 28 extending down into well bore 11 to below the minimum level 12 of the water table top 37 of rod 18 abuts against window 29 spring 38 is positioned in a compressed condition between spring bearing 26 and spring bearing ledge 30 to provide biasing means for maintaining rod 18 in compression and sleeve 19 in tension sight ports 35 and 36 are positioned and directed to look at each other across the interior of housing 15 so that an optical path is completed between optic fibers 41 and 42 when rod and sleeve assembly 17 is not installed in housing 15 but is not completed when rod and sleeve assembly 17 is installed in housing 15 sight ports 33 and 34 are positioned and directed to look at each other across the interior of housing 15 so that an optical path is completed between optic fibers 39 and 40 when only rod 18 occupies the upper part of housing 15 but is not completed when top fitting.