Some years ago (1) the author described a recording potentiometer designed primarily to meet the requirements of flexibility so necessary in physiological work. A number of these instruments were built and are still in use. Ever since this earlier instrument was developed the writer has continued to improve and simplify until now one which is far superior in all respects to this first instrument has been constructed. This instrument is fundamentally a self-balancing potentiometer controlled by radio tubes. It differs from the earlier model, however, in being much more sensitive, stable, compact, and particularly mechanically reliable. All parts used in its construction are standard and only a minimum amount of machining is