consequence of the commercial importance that the electric transmission of energy now possesses. And it may be mentioned that generally where problems of maxima or minima have been considered, attention has been directed to the kind of change that is produced in the value of the quantity under consideration, when the value of the variable is altered from that required to make the quantity a maximum or a minimum. In fact, the aim has been to treat a few subjects fairly thoroughly in a simple manner, and not to prepare a list of short instructions for carrying out a large number of experiments, nor to write a treatise, mainly of value as an electrical dictionary, which should give a little information about everything that can be comprised under the head of electricity, whether it be electric eels, the history of the invention of the telegraph, the aurora, or the earliest forms of frictional machines. In the letterpress, small capitals have been used to represent instruments, parts of apparatus, &c., while large capitals systematically stand for electric quantities. other than resistances, these being throughout designated by small letters in italics. Thus A, A, a stand respectively for an ammeter, the current in amperes flowing through it, and its resistance in ohms. In the preface written in 1886 it was mentioned that, with the exception of two or three blocks that had been lent, the 180 figures had been specially drawn for the book, and were not time-honoured representations of historical apparatus. Of these 180 figures only 64, however, have been employed in the present volume, partly because the fresh matter required many new figures to illustrate it, and partly because several of the blocks specially executed for the original book have lost their freshness from the appreciative use of them by other writers. Hence, 183 of the 247 figures contained in the present volume will not be found in the former book, and 163 of these fresh illustrations have been specially drawn for this new edition. A large number of new examples have been added, and any that have been reproduced from the original book have been reworked, either to check the accuracy of the results, or because the so-called legal units referred to have been replaced by those that have now been adopted internationally. My thanks are due to my past and present assistants -Dr. Sumpner, Mr. Haycraft, and Mr. Severs-for much assistance in the preparation of this book; to my daughter for compiling a very comprehensive and judiciously arranged index; and to Messrs. Spiers, Twyman, and other students for carefully examining the proofs. In conclusion, I desire to express to-day even more warmly than in October, 1886, my indebtedness to Mr. Mather for the very earnest, thoughtful, and painstaking way in which for many years he has assisted me in developing the course of instruction for students of electrical technology, of which the present volume represents part of the elementary portion. October, 1896. W. E. AYRTON, TABLE OF CONTENTS. 5. The Strength of an Electric Current: by which of its Pro- perties shall it be Directly Measured? 6. Definition of the Unit Current; Ampere 7. Definition of the Direction of the Current 8. Objection to the Usual Mode of Constructing Voltameters 9. Description of Practical Forms of Sulphuric Acid Volta- 10. Relative Advantages of Voltameters and Galvanometers 11. Meaning of the Relative and the Absolute Calibration of a 14. Practical Value of Drawing Curves to Graphically Record 15. To Construct a Galvanometer Scale from which the Relative 18. Why Magnetic Needles tend to Point North and South 26. Variation of the Sensibility of a Tangent Galvanometer with the Number of Windings, and with the Diameter of the 27. Values in Amperes of the Deflections of a Tangent Galvano- 32. Employment of the Sine Principle in Galvanometers 33. Construction of Galvanometers in which the Angular Deflec- tion is directly Proportional to the Current 34. Galvanometers of Invariable Sensibility DIFFERENCE OF POTENTIAL, AND RESISTANCE. 56. No Force Inside a Closed Conductor Produced by Exterior 57. Potential due to Exterior Electrostatic Action is Uniform at 60. The Potential of a Body Depends Partly on its Position 61. The Potential of a Body Depends Partly on its Size and 62. The Potential of a Body Depends Partly on a Third Condi tion the Quantity of Electricity 63. No Electricity at Rest Inside a Conductor 67. Testing the Sign of the Electrification of a Body 68. Screening Outside Space from Inside Electrostatic Action 78. Use of a Shunt with the Bridge 79. Meaning of the Deflection on a Bridge Galvanometer |