A pair of ordinary weighing scales also exemplifies the first
order of levers. The knife edge on which the beam is balanced serves as
a fulcrum; it is placed exactly in the middle of the beam, which we
shall suppose to be 10 inches long. If we place a 1-lb. weight in one
scale to represent the resistance to be overcome, the weight will be
lifted the moment that a pound of sugar has been placed in the opposite
scale--the sugar thus representing the power. If, however, we move the
knife-edge or fulcrum so that it is only 1 inch from the sugar end of
the beam and 9 inches from the weight end, then we find that we have to
pour in 9 lb. of sugar to equalise the 1-lb. weight. The chisel used in
prying open the box lid was 10 inches long; it was pushed under the lid
for a distance of 1 inch, leaving 9 inches for use as a power lever. By
using a lever in this way, we increased our strength ninefold. The
longer we make the power arm, the nearer we push the fulcrum towards the
weight or resistance end, the greater becomes our power.
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