PREFACE.I HAVE often been asked to describe my method of making a capillary electrometer, and my apparatus for the measurement of rapid changes of E.M.P. My aim in this series of articles, written in October, 1895, has been to do so with sufficient fitness to enable anyone possessed of average skill to construct the instrmnent. I have laid most stress on the production and interpretation of the photographic records as illustrating the special charactcristics of the capillary electrometer but it is hardly necessary to point out that it may be used by direct observation with the Wheatstone bridge, or with the potentiometer, instead of a high-resistance galvanometer, and that it may replace the quadrant electrometer in comparing the capacities of condensers. In response to a generally expressed wish, these articles havc been reprinted, as a preliminary to the more complete work upon the subject which I hope to write. The capillary electrometer was invented by Lippmann, and described by...