It has been planned to present the material in such a manner that it will aid the judgment of the young experimenter and assist him in developing his own ideas. Without exception, all of the apparatus described in the following pages has been actually constructed by the author, not only once but many times and put to a practical test before being embodied into the book. You may therefore be sure that if you follow the instructions carefully, that the result will in each case be a substantial piece of apparatus which is capable of fulfilling all of your expectations. The drawings have all been reproduced on a large scale and in almost every case the dimensions of even the smallest details have been given. Some of the apparatus has been described in the pages of the "Boys’ Magazine" and since its publication the readers of that magazine have written to the author asking questions about the apparatus which have enabled him when rewriting the material for publication in book form to clear up many questions and further explain in a little more detail many of the problems which naturally occur to the boy who likes to build his own electrical devices. THE AUTHOR. PREFACE ........................................................... CHAPTER I. STATIC ELECTRICAL APPARATUS ............................ How to Build a Wimshurst Machine. ............................... Experiments with Static Electrical Apparatus. ................... CHAPTER II. CELLS AND BATTERIES. .................................. The Voltaic Cell. ............................................... Homemade Batteries. ............................................. Battery Solutions or Electrolytes. .............................. Connecting Cells. ............................................... Storage or Secondary Cells. ..................................... An Experimental Storage Cell. ................................... A Homemade Storage Cell. ........................................ Recharging and Caring for Storage Cells. ........................ CHAPTER III. HOW TO REDUCE THE 110 V. D.C. OR A.C. TO A LOWER VOLTAGE FOR EXPERIMENTAL PURPOSES. ................................ CHAPTER IV. HOW AN ALTERNATING CURRENT MAY BE CHANGED INTO DIRECT CURRENT BY MEANS OF AN ELECTROLYTIC RECTIFIER. .................... CHAPTER V. HOW TO BUILD A STEP-DOWN TRANSFORMER FOR REDUCING THE 110 VOLT A. C. FOR EXPERIMENTAL PURPOSES. ............................. CHAPTER VI. ELECTRIC MEASURING INSTRUMENTS ........................ Galvanometers, Ammeters, Voltmeters. How to Make a Galvanometer. . The Construction of Ammeters and Voltmeters. .................... CHAPTER VII. CURRENT CONTROL DEVICES. ............................. How to Make a Pole Changing Switch or Current Reverses How to Reverse a Small Motor. ..........................................