Capacitance of an electrode covered in filler media (e.g., air or oil),
        normalized per unit area (i.e., units are F/mm^2).

def c_filler(self, frequency):
        '''
        Capacitance of an electrode covered in filler media (e.g., air or oil),
        normalized per unit area (i.e., units are F/mm^2).
        '''
        try:
            return np.interp(frequency,
                             self._c_filler['frequency'],
                             self._c_filler['capacitance']
            )
        except:
            pass
        return self._c_filler