text: Try sound.  The musical scale is a log base 2 scale (A = 440, and 880, and 220), while sound loudness is logarithmic, that's why they use decibels.  Double the power output from an amp and you have a (log of 2 = 0.3, times ten =3dB gain).  Look at the volume control on an amp or radio, its logarithmic, not linear.  Also, try the Beaufort wind scale, this is based on effect, not on a linear scale; earthquakes (again, based on effect); Clouds (no scale at all).  \nTo make it easy why not start with earthquakes.  This is an effect based scale so the students will be able to assess easily the effect on the local population.