entire working range i e between their expected maximum and minimum lengths to support the weight of bottom rail 116 and any accumulated slats 112 taking into account any frictional forces in the system so that bottom rail 116 does not free fall when released however extension springs 136 and 138 should not be selected to provide a tension force that is so strong that bottom rail 116 moves upwardly on its own accord when released by selecting springs of the appropriate strengths and or manipulating the frictional forces in blind system 110 the blind system can be properly balanced so that bottom rail 116 reliably remains in the position to which it is urged according to a well known equation known as hooke 39 s law the force that an extension spring exerts on a mass is directly proportional to its extension and always acts to reduce this extension where f is the spring force k is a positive quantity called the force constant of the spring and u03b4 is the change in length or extension of the spring hence it will be noted that the spring force f provided by extension springs 136 and 138 increases as bottom rail 116 is lowered because lowering bottom rail 116 results in further extension of springs 136 and 138 as persons skilled in the art will recognize this provides a force curve that is precisely opposite what would be ideal because springs 136 and 138 are required to do less work as bottom rail 116 is lowered as a result of less slats being accumulated thereon accordingly to properly.