_Where recourse is had to the question and answer form, the special aim of the author has been to give short and direct answers, in such plain language as to preclude a misconception of the meaning. With this in view, =the answer gives= simply =the information sought by the question=._ _=The answer is limited to one paragraph= so that the reader may concentrate upon the fact or facts demanded by the question._ _Any enlargement of the answer or specific explanations of items contained therein, are presented in separate paragraphs printed, in smaller type._ _With this plan of =separating the answer=, as it were, from items of secondary importance, and making it short and simple, its content is more forcibly impressed upon the mind of the reader._ _In a text book, it is necessary to illustrate and explain the various species of commercial apparatus met with in practice, and in this connection the Publishers desire to call attention to the manner in which the author has treated what may be classed as the =“descriptive matter.”= Contrary to the usual custom of giving descriptions of commercial machines in the main text, where they would occupy considerable space, to the exclusion of the more important matter, all such descriptions are placed in small type directly under the illustrations, leaving space for an adequate presentation of the underlying =principles=, =theories=, and for the large amount of =practical information= that is essential to obtain a general knowledge of Electricity and its numerous applications._ _Credit is largely due to Frank D. Graham, B.S., M.S. (Princeton University), and M.E. (Stevens Institute), practical engineer, for the authorship of the =Guides=, and for original sketches illustrating electrical principles and construction._ TABLE OF CONTENTS GUIDE NO. 1. =INTRODUCTORY CHAPTER= =SIGNS AND SYMBOLS= =ELECTRICITY= =1 to 4= Nature and source--=kinds of electricity:= static, current, dynamic, radiated, positive, negative, atmospheric, frictional, resinous, vitreous. =STATIC ELECTRICITY= =5 to 26= Electrical =attraction and repulsion=--the charge--distribution of the charge--free and bound electricity--conductors and insulators--=electroscopes=--gold leaf electroscope--electric screens--electrification by induction--nature of the induced charge--the electrophorus--=condensers=; Leyden jar--=electric machines=--action of Toepler-Holtz machine--Wimshurst machines. =THE ELECTRIC CURRENT= =27 to 34= Volt--ampere--ohm--=Ohm’s law=--production of the electric current--current strength--voltage drop in an electric current. =PRIMARY CELLS= =35 to 67=