Capacitance of an electrode covered in liquid, normalized per unit
        area (i.e., units are F/mm^2).

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