NATURE 



537 



THURSDAY, OCTOBER 7, 1897. 



PRACTICAL ELECTRICITY. 

 Practical Electricity : a Laboratory and Lecticre Course 

 for First Year Students of Electrical Engineering, 

 based on the International Definitions of the Electrical 

 Units. Completely rewritten by W. E. Ayrton, F.R.S. 

 \'ol. i. Current., Pressure., Resistance, Energy, Power, 

 and Cells. Pp. xviii + 643, with 247 illustrations. 

 (London : Cassell and Co., Ltd., 1896.) 



THAT the progress of electricity during the last 

 decade has been great appears in many ways ; but 

 none shows this progress more strikingly than a com- 

 parison of a new and up-to-date book on practical elec- 

 tricity with the work as it originally appeared ten years 

 ago. Prof. Ayrton's book was well-known to all laboratory 

 students of electricity ; it contained in moderate compass 

 a vast amount of detailed information of great value, and 

 it had marched well with the times. But the decision of 

 the author to recast his matter, and to issue the book in 

 an entirely rewritten and rearranged form, was none the 

 less a wise one. He has now freer scope to put things in 

 their newer relations and lights, to introduce new matter, 

 and to emphasise those points and topics which still 

 further increased experience and the trend of applications 

 show ought to be more insisted on, and ought to bulk 

 more largely in a thoroughly modern treatment of 

 the subject. 



So far only one volume of the new book has been 

 published, and that deals with current, pressure, resist- 

 ance, energy, power, and cells. The treatment of these 

 topics, it is needless to say, is fresh and vigorous through- 

 out. The individuality of the author is evident in every 

 page, and however we may differ in opinion from him on 

 such subjects as order of discussion, choice of starting 

 points, and the like, there can be no question that for 

 every position taken up, and every deviation from the 

 ordinary routine of an elementary treatise on electricity, 

 tlie author is provided with a reason which commands 

 attention and respect. 



In the brief review which we here propose to make of 

 this volume, we shall first note some points that have 

 struck us in a general survey of its contents, and then 

 allude more particularly to some matters of detail. First, 

 there is the very large number of clearly drawn and 

 well-chosen cuts with which its pages are illustrated. 

 There are 247 in all, about one to every two pages and a 

 half, and about three out of every four of them have been 

 drawn for the present work. This feature of the book is 

 refreshing as contrasted with what one finds in so many 

 text-books, which seem to be to a large extent written 

 up (?) to a somewhat well-worn set of " process " cuts. 



Some of the pictures of instruments are beautiful ex- 

 amples of what such illustrations should be. If a shaded 

 diagram is given, it should be clear, and not, as it often is, 

 a more of less smudged kind of impressionist representa- 

 tion of the instrument. Then, essential parts hidden away 

 behind coils or in tubes should in some way be disclosed, 

 and not left to be imagined from a verbal description. 

 Thus, in the drawings of current-meters, in various parts 

 of the book, the device of showing the coils cut open to 

 NO. 1458. VOL. 56] 



display the construction, and to reveal suspensions and 

 suspended coils and needles, is adopted with great effect. 

 Some of the cuts of lines of force, which have evidently 

 been engraved from photographs of the actual arrange- 

 ments, strike us as the finest we have seen ; notably one 

 (Fig. 34) of the lines of force in a plane through the 

 axis produced by a circular coil carrying a current, and 

 two (Figs. 57, 57 rt) of the lines of force of a horse-shoe 

 magnet with curved iron pole-pieces. These last are 

 exceedingly instructive as regards concentration of field 

 and remanent leakage of lines outside the space. 



The printing and get-up of the book are all that can be 

 desired. Printed on thin and at the same time opaque 

 paper, it contains 600 pages of text within a bulk not 

 larger than that often given to a book of little more than 

 half the same amount of matter. A clear and well- 

 arranged index has been provided by Miss Ayrton. 



Now as to the particular points in the author's treat- 

 ment, regarding which we would say a word. The book 

 is professedly based on the international definitions of 

 the various units. Hence the discussion of currents 

 begins with the enumeration of the various physical 

 actions of a current, and a decision is come to, the con- 

 venience of which is shown in various ways, to regard 

 an unvarying current as proportional to the chemical de- 

 composition it produces in a given time. The unit of 

 current is defined SiS that which passed through a solution 

 of nitrate of silver in water deposits silver at the rate of 

 •001 1 18 of a gramme per second. Then is given in a 

 very lucid manner much valuable information regarding 

 electrolysis and the quantitative results that have been 

 arrived at in the investigation of the electrolysis of 

 silver and copper, and the electrolytic standardisation 

 of current-measuring instruments. 



Now, although the units referred to above have been 

 the subject of international agreement, their adoption is 

 a matter of practical and commercial convenience, and 

 should not be allowed to supersede or put out of sight 

 the direct derivation of the units of current E.M.F., &c., 

 in the absolute C.G.S. system of units. An Order in 

 Council is not a law of the Medes and Persians which 

 cannot be changed ; on the contrary, it will have to be 

 altered in any respect in which, under the conditions of 

 future practice, it is found to be too much at variance 

 with later and more accurate determinations of electrical 

 constants, in order to bring it into agreement with the 

 absolute system. The derivation of the absolute unit of 

 current from the magnetic action of a current has a 

 great practical as well as theoretical importance, and 

 constitutes, as it seems to us, the ox\\y proT^^r definition 

 of unit current. Of course, this definition neces- 

 sitates, in the first place, a discussion of magnetic effects 

 of currents ; but this cannot be avoided, and is, of 

 course, very adequately given later in Prof. Ayrton's 

 treatise. But we prefer that it should come before the 

 electrolytic discussion, as without a familiarity with the 

 specification of the measure of a current by its magnetic 

 action the full force of the results of electrolytic research 

 cannot be quite appreciated. But this procedure adopted, 

 the results of such research connecting currents measured 

 by their magnetic action with their electrolytic effects 

 comes in what seems to us both their natural and their 

 historical place, and the choice of '001118 gramme of 



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