of time to the preparation of what in reality is an entirely new book, although called by its old title "PRACTICAL ELECTRICITY."

The reception of the first edition took me by surprise. I anticipated that the book would be regarded as "faddy," and that the critics, while admitting that perhaps it would do well enough for my own classes, would not recommend its use for students in general. It did not occur to me that the world was ready for using such a text-book and prepared to adopt the methods of teaching advocated in its pages. To-day, however, the following reasons suggested in the original preface for even elementary students of electricity spending much time in the laboratory would be advanced by many teachers :

"One of the great difficulties experienced by people in mastering the quantitative science of electricity, arises from the fact that we do not number an electrical sense among our other senses, and hence we have no intuitive perception of electrical phenomena. During childhood we did not have years of unconscious experimenting with electrical forces as we had with the forces connected with the sensations of heaviness and lightness, loudness and softness, heat and cold. Beyond a shock or two taken perhaps from some medical galvanic apparatus, or from a Leyden jar, our senses have never been affected by electrical action, and hence we ought to begin the study of electricity as a child begins its early education. Quite an infant has distinct ideas about hot and cold, although it may not be able to put its ideas into words, and yet many a student of electricity of mature years has but the haziest notions of the exact meaning of high and low potential, the electrical analogues of hot and cold. That it is desirable that students should learn physics, as they learn to ride the bicycle, by experimenting themselves, is now generally admitted, and this is especially true in the case of electricity, since it is by experimenting, and only by experimenting, that a student

can obtain such a real grasp of electricity that its laws become, so to say, a part of his nature."

"Hence, in the courses of electricity which I arranged at the City and Guilds of London Technical College, Finsbury, and at their Central Technical College, Exhibition Road, for every hour that a student spends at lecture, he spends several in the laboratory."

When Dr. Hopkinson this year, 1896, in his Inaugural Address as President of the Institution of Electrical Engineers, advocated commencing the study of electricity with the electric current, more than one teacher testified to the value of the method by claiming it as his own, apparently forgetful that when this order of treating the subject was introduced by the author in 1879 there was no precedent for such an innovation. Indeed, when even seven years later there appeared the first edition of "PRACTICAL ELECTRICITY" it was thought advisable to introduce the method by inserting the following explanatory paragraphs :

"Readers who have been accustomed only to the ordinary books, commencing with certain chapters on statical electricity, continuing with one or more on magnetism, and ending with some on current electricity, will be surprised at the arrangement of the subjects in this book, and will probably be astonished at what they will condemn, at the first reading, as a total want of order. But so far from the various subjects having been thrown together hap-hazard, the order in which they have been arranged has been a matter of the most careful consideration, and has been arrived at by following what appears to me to be the natural as distinguished from the scholastic method of studying electricity. I have endeavoured to treat the subject analytically rather than synthetically, because that race of successful experimental philosophers -children-adopt this method.

"For example, it is not by studying geometrical optics, much less physical optics, that an infant gradually learns to appreciate the distance of objects; and later on it is

not by studying a treatise on struts, nor by listening to a course of lectures on structures, that the child finds out that the table has legs, hard legs, round legs. Feeling, looking, trying, in fact a simple course of experimental investigation, gives a child its knowledge; and this, therefore, I venture to think, is the method we should adopt when commencing the study of electricity."

"The subject of current is treated first, because in almost all the industries in which electricity is practically made use of, it is the electric current that is employed; also, because currents can be compared with one another, and the unit of current (the ampere) defined, without any knowledge of potential difference or resistance. Potential difference is next considered, and resistance the last of the three, because the very idea of resistance implies a previous acquaintance with the ideas of current and potential difference, since the resistance of a conductor is the name given to the ratio of the potential difference (measured electrostatically) between its terminals to the current passing through it. And it is Ohm's experimental proof that this ratio was constant for a given conductor under given conditions, together with the numberless experimental verifications that this conclusion has received, that has led to resistance gradually coming to be considered as a fixed definite property of a given conductor like its weight or length."

The international, or Board of Trade, unit of P.D.— the volt―cannot, however, be defined until the definition of the unit of resistance—the ohm—has been fixed, because for legal purposes the units of current and resistance have been taken as the primary ones, and Ohm's law has been employed to fix the third or derived unit—viz. that of P.D. Hence, the actual sequence adopted in the present volume is (1) current and the ampere; (2) the relative measurement of P.Ds. with a zero electrostatic voltmeter; (3) Ohm's law; (4) resistance, and the ohm; (5) the volt, and current-voltmeters. Electric

energy and power, with their units-the joule and the watt-are next treated; and, lastly, the conception of the E.M.F. in a circuit, and the necessity for the E. M.F. of a good cell being constant, are derived from the laws of energy.

It should be obvious that any method of trying to experimentally prove Ohm's law with a current-voltmeter, such as may be found in certain text-books, begs the question. If a voltmeter be used it must be of the electrostatic type, and to simplify the definition of one P.D. being twice another this electrostatic voltmeter should be a zero instrument, which, without the need of any independent electrification, would be suitable for measuring P.Ds. no larger than those commonly employed in laboratories for sending currents. Such an instrument I have long felt the need of, and now-thanks to the ingenuity of Mr. Mather-it is available for use, and will be found described for the first time in pages 163-166 of the present volume.

It will be observed that the apparatus required for each experiment is mounted complete on a board. This is to enable it to be easily carried backwards and forwards between the laboratory and the lecture-room without disarranging it. At first sight it might appear that the student, finding each set of apparatus joined up quite complete, with current laid on all ready for carrying out the experiment, would be deprived of all incentive to exercise his own ingenuity in overcoming experimental difficulties, and, therefore, would fail to acquire habits of self-reliance. For first year students, however, I have found it a good plan to have each set of apparatus complete in position; firstly, because it is only with some such arrangement that fifty or more students can commence work almost simultaneously, and in the course of two or three hours have all performed some quantitative experiment; secondly, because when the apparatus is so arranged that even beginners can perform several experiments successfully, they acquire faith in the

possibility of success, and are less discouraged with the difficulties they subsequently meet with when selecting and arranging the apparatus for conducting some investigation.

The practical side of electricity has grown so rapidly that the original single volume has expanded into two. The present Volume I. of the rewritten book is intended to assist students in acquiring experimentally an exact working knowledge of current, difference of potentials, resistance, energy, and power, with their electric transmission, cells and their cost of working. This subject of the cost of converting chemical energy into electric energy is not, as far as I am aware, to be found in any text-book. Hence, in view of the "booms" in primary batteries, which appear to be periodic, the question of cost has been entered into in considerable detail.

The past four years have seen the legalisation in several countries of an international system of electrical units, so that, while the units of length, volume, mass, and money vary from country to country, there is now but one ampere, one ohm, and one volt throughout the whole world; a fact of which electrical engineers may feel justly proud. Some thirty pages at the end of the book are, therefore, devoted to "A Short History of the Absolute Unit of Resistance, and of the Electrical Standards of the Board of Trade."

In spite of the fact that the present Volume I. contains some 140 pages more than the original book, the subjects of secondary cells, electric quantity, coulombmeters, capacity, &c., have had to be left for a second volume. This has arisen not merely from primary cells, including dry cells and the Clark's standard cell, having been treated somewhat fully, but from the subjects of electric energy and power, the various meters used for measuring these quantities, the efficiency of electric transmission, the ratio of the power received to the maximum power receivable in various cases of transmission, &c., having been entered into at length in