text: The force measurements of the SFA are based primarily on Hooke's Law,
  where F is the restoring force of a spring, k is the spring constant and x is the displacement of the spring.
  Using a cantilevered spring, the lower surface is brought towards the top surface using a fine micrometer or piezotube. The force between the two surfaces is measured by
  where is the change in displacement applied by the micrometer and  is the change displacement measured by interferometry.
  The spring constants can range anywhere from  to . When measuring higher forces, a spring with a higher spring constant would be used.