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 him in a thesia published in 1875. Being specially adapted for the study of electromotive changes of short duration, it attracted the attention of physiologists. In 1877 IIarey employed it in investigating the functions of the electrical organ of the torpedo, and succeeded in obtaining photographic record of its indications. In England Prof. Burdon Sanderson used it first in a research by which he proved that the closure of the leaf of tho Venus Fly Trap ia preceded by electrical changes resembling those mhich accompany the contraction of animal muscles. In a lecture delivered before the Royal Institution in 1882 he exhibited photographs of the excursions of the capillary electrometer produced in this way...