For the past few years the writer of this book has given a course of lectures on the elementary theory of experimental electricity to a class of students at Trinity College, Cambridge. Experience, together with an occasional study of lecture notebooks, has indicated that, to supplement the lectures, some definite and permanent statement is required - some book of reference, to which the students may turn for further elucidation of points not clear to them.<br><br>Thus the alternatives arose either of adapting the lectures to the lines of treatment of an existing book, or of writing a book which should correspond with the stage now reached in the evolution of teaching, which has extended over some ten years.<br><br>The great shift in the chief points of interest of experimental electricity, due to recent development in physical science, has changed the proportion of the various branches of the subject, and has put out of date many of the older standard text-books. To the phenomena of electrolysis, of conduction through gases, and of radio-activity, the physicist will now turn for knowledge newly acquired, for knowledge in the making, and for unsurveyed territory ready and waiting for the explorer.