The present work deals with Hall effect and specific conductivity measurements in the case of a nonuniform current density field (that is, when the specimen is not a parallelepiped but has irregularly waved surfaces). It is then impossible to measure separatelyRH(Hall coefficient) and &sgr; (conductivity); it is possible, however, to get their product, the Hall mobility of the carriers. These conclusions are important when dealing with thin films, where the actual geometry of the surface is completely unknown. The preceding remarks are used to interpret the results of some experiments on Au films evaporated on mica. An almost constant mobility is foundversusmean thickness, but its magnitude is about ¼ the bulk mobility. This behavior raises some question about the structure of thin films, and some connection seems to hold between the experiments here described and the results of the measurements of the optical constants of metals.