Measurements of the influence of stress upon contact potential necessitate the observation of shifts in contact potential of the order of 100 mgr;V or less. The ac Kelvin technique was selected to yield this sensitivity. The application of phase sensitive detection techniques to the ac Kelvin method is discussed, and the equations are solved. The success of the method depends upon the use of a guarded field effect transistor input stage located within the Kelvin probe. The effect of nonuniform contact potentials and nonuniform capacitor plate separation is analyzed, and conditions are given for the balance point to be independent of plate spacing. The stress dependent contact potential measurements may be continuously displayed using a new nonlinear second harmonic detection scheme. This method is discussed.
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