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Two frequencies are simultaneously applied to a vibrating capacitor for making contact potential difference measurements. Since the potential difference across the capacitor is usually a nonlinear function of the electrode spacing the resultant signal consists of one frequency modulated in amplitude by the other. By detecting only this amplitude modulation it is possible to observe contact potential differences while one of the surfaces, forming the capacitor, is emitting electrons or ions. Also the reproducibility of contact potential difference measurements is increased by a factor of ten over the more usual single frequency method. The system uses integral feedback to provide a self-balancing facility. The temperature coefficient of the minimum work function for a caesiated surface was found to be − 1·2±0·2 × 10 −3 eV degK −1 over the temperature range 300 to 850°K. The work functions of tungsten (100) and (110) surfaces were measured to be 4·645±0·005 eV and 5·141±0·007 eV respectively, assuming that of the polycrystalline reference surface to be 4·550 eV.
Blott et al. (1969) studied this question.