l62 



NATURE 



[October 31, 19 18 



portraits is supposed to recall the founders of 

 modern chemistry and the contributors of tunda- 

 1 ideas one would expect to find the tans ol 

 Cannizzaro, Frankland, Raoult, Ramsay, Crookes, 

 and perhaps several others whose services were al 

 Last equal to those ol some of the chemists 

 repi esented. 



The book will be found useful b) many students 

 and by older chemists who ma) wish to refresh 

 their memories of the origin and progress of ideas 

 in the science to which the) are devoted. 



W. A. T. 



ELECTRICAL BOOKS FOR STUDENTS. 



(r) Alternating-current Electrical Engineering. 



Bv Philip Kemp. Pp. xi + 494. (London: Mac- 



millan and Co., Ltd., 191S.) Price 17s. net. 



(2) Magnetism and Electricity for Home Study. 



By 11. E. Penrose. Pp. xxiii + 515. (London: 



The Wireless Press, Ltd., 1918.) Price 5s. net. 



(1) T N his preface the author states that this 



i text-book "covers in a general way the 



main ground included in the title without going 



into too great detail in any one particular branch." 



It is intended for engineers and students, and 



covers the syllabus for the Grade II. (A.C.) paper 



of the City and Guilds examination. So wide is 



the field covered that the book is almost an 



encyclopaedia of alternating-current practice. The 



general reader, therefore, will be disappointed in 



places owing to the brevity of the descriptions 



and the lack of full explanations. In a book 



written mainly for examinational purposes there 



is not much room for original matter, but some 



of the proofs are neat and novel, and the author 



gives a new system of harmonic analysis which 



we discuss below. 



To illustrate the danger of brief explanations, 

 let us consider the definition of a condenser given 

 on p. 50. This produces a blurred mental 

 picture. "When two conducting bodies are 

 separated by a dielectric they are said to possess 

 capacity, and the combination is called a con- 

 denser." We are not told whether the conducting 

 bodies are equal or not. When a difference of 

 potential is applied we are told that one of the 

 "plates" is positively, and the other negatively, 

 charged. It is not stated definitely whether the 

 positive charge is equal numerically to the nega- 

 tive charge, or what exactly is meant by the 

 charge in a condenser. The descriptions of the 

 Mansbridge and Moscicki condensers do not help 

 the reader to clear up this point, and neither do 

 the analogies given between a condenser and a 

 vis globe, and between a condenser and a rubber 

 diaphragm in a lube. 



In the chapter on wave form several methods 

 of harmonic analysis an given. In the first 

 method Fourier's constants are determined by a 

 simple but somewhat laborious evaluation of 

 Fourier's integrals by mechanical quadrature. 

 The method involves some very tedious calcula- 

 tions when high harmonics are r< quired, but it is 

 theoretically sound. In the author's method the 

 '■ 2557, VOL. I02~ 



assumption is made that the alternating wave 



can lie expressed exactly by an equation ol thl 

 form : — 



e = Ej sin (8 + a,) + E 3 sin (3$+ a 3 )+ . . . 4- 



K| 7 sin (176+ a 17 ). 



It is then shown thai by measuring eighteen ordi- 

 nal. :s ol the positive half of the wave at the points 

 K5, 25, . . . 175 deg. respectively and proceed- 

 in a perfectly regular and simple wa) wi 

 irmine all the values ol K,, 1\„, E 6) . . . E17 and 

 otj, a. ( , a 5 , . . . a 17 . A schedule is given by filling 

 up the blanks by which anyone can find these 

 values without excessive labour. This solution is 

 vvorthv of commendation, as it is much superior 

 to that given by Runge, modifications of which 

 are in everyday use. It has to be pointed out, 

 however, that the author's solution is strictly only 

 an interpolation formula. The Fourier constants 

 are the constants we want in harmonic analysis, 

 and it is highly probable that in most cases the 

 higher harmonics determined in the author's way 

 will only be rough approximations. 



(2) In a series of fifty lessons the author dis- 

 cusses the elements of electricity and magnetism 

 and also electrical engineering. We learn from 

 the introduction that during the war he has given 

 instruction in these subjects to a large number of 

 recruits for the Royal Flying Corps, the Royal 

 Navy, the Transport Service, and the Mercantile 

 Marine. There is no doubt that the ordinary 

 academical method of teaching the subject repels 

 many, and so most teachers nowadays — espe- 

 cially those who are not cramped by an exam- 

 inational syllabus — stimulate the enthusiasm of 

 their pupils by making excursions into the 

 imaginative realms of modern theory. For 

 students of radio-telegraphy this practice is to be 

 highly commended, as familiarity with these 

 theories will be of real assistance when they have 

 to work with valves and rectifiers. The drawback 

 to introducing these theories at such an early stage 

 is that it is bound in many cases to give rise to 

 extraordinary misconceptions. To be told that a 

 molecule must be regarded as a universe com- 

 posed of several miniature solar systems stretches 

 the imagination, but to accept it without proof 

 and without any statement of its limitations is 

 to expect the reader to adopt an altogether too 

 passive attitude. 



There are some strange errors in the book. We 

 are told, for instance, that the existence of the 

 asther is proved by the "lines of force surrounding 

 a magnet in a vacuum "• (p. 23). It is stated on 

 p. 20S that the resistivity of a wire is not affected 

 by increase of temperature, although the resist- 

 •ance is. We are also told (p. 303) that if we 

 have two spheres, the surface of one of which is 

 double that of the other, and if they be charged 

 from the same source, then it is "fairly evident" 

 that the charge on one will be double that of the 

 other. Various deductions in capital letters are 

 made from this, which preclude the suggestion 

 thai "surface" is a misprint for "radius." The 

 writing is spirited in places, but the words are 



