An electro-optical device is described which provides a continuously-variable dc electrical signal in response to a shaft rotation. The elimination of a mechanical contact provides low torque, long life, and high-speed operation. Shaping of the thin film resistance track associated with this device produces an interesting function generator. Data is presented for successful operation of the laboratory model. An analysis is provided of the requisite track shaping to produce arbitrary functions. The electrical equivalent circuit of the device is analyzed, and the conditions necessary for successful operation described.