Abstract: An apparatus for measuring changes in the work function of a surface using a method devised by Kelvin and known as the "dynamic capacitor method". This apparatus comprises a system for monitoring and automatically regulating the vibration of a Kelvin probe reed assembly during variation of the experimental conditions. This system includes a piezo-ceramic crystal mounted on the probe support rod to detect the vibrations of the reed and a phase locked loop circuit which compares the signal from the piezo-ceramic detector with the signal of the reed drive coil, interprets a change in the reed resonance as a phase shift relative to the coil drive phase and then shifts the coil drive frequency to maintain the correct phase relationship.