docs: def c_drop(self, frequency):
          '''
          Capacitance of an electrode covered in liquid, normalized per unit
          area (i.e., units are F/mm^2).
          '''
          try:
              return np.interp(frequency,
                               self._c_drop['frequency'],
                               self._c_drop['capacitance']
              )
          except:
              pass
          return self._c_drop