NA TURE 



217 



THURSDAY, JANUARY 5, li 



ELECTRICITY FOR PUBLIC SCHOOLS AND 

 COLLEGES. 



Electricity for Public Schools and Colleges. By W. 

 Larden, M.A. (London : Longmans, Green, and Co., 

 1887.) 



THIS is a book which possesses many good points, 

 but which becomes, on close acquaintance, pain- 

 fully disappointing, and even irritating, to the reader. The 

 author has undoubtedly spared no pains to make it full 

 of information ; but its very fullness becomes bewildering, 

 owing to the way in which the material is cut up and put 

 together. One might almost imagine that it had been 

 reduced from a much larger work, chiefly by means of 

 deletions, and without the rounding-off of the angularities 

 which such a process would inevitably develop. 



The science of electricity and magnetism is, without 

 question, an experimental science ; and the author of the 

 present work does not offer his book to his readers as 

 a book on the mathematical side of this experimental 

 science ; but as an elementary book suitable for higher 

 schools and for Colleges. He confines himself as to 

 mathematics by assuming " no more mathematical know- 

 ledge than is usually possessed by the higher boys in a 

 classical school." Under these conditions we should 

 expect to have a book containing exact and well-finished 

 descriptions of experiments and apparatus, along with 

 explanations of phenomena and with theory brought down 

 to correspond with our present knowledge on this most 

 fascinating subject. Expectations or hopes such as these 

 are very far from being realized in the book before us. 



The amount of material collected by the author is 

 undoubtedly very great. The number of instruments 

 and machines referred to and described is enormous. 

 The descriptions are, however, often unsatisfactory, nor 

 are they written with any attempt at finish or good taste 

 The book is supposed to be for the use of well-educated 

 beginners ; but we think it would be difficult to find a 

 worse model for boys or young men as to the writing of 

 descriptions of apparatus or of experiments. Often the 

 heading of a paragraph has half the duty to perform ; 

 the remainder may be done by a diagram, which is let- 

 tered in a tantalizing way, as if a description had been 

 intended. 



Here, for example, are the descriptions of two of the 

 most important frictional electric machines : — 



" IL The Common Plate Machine. — In this there is 

 nothing essentially different from the cylinder machine. 

 A glance at the figure will explain all. There are gener- 

 ally two rubbers ; and in this form of machine they can- 

 not well be insulated, if required ; so the machine cannot 

 be used as a source of both -|- and — electricity. In- 

 stead of glass, ebonite plates may be used, the rubbers 

 being of amalgamated silk. 



" III. Winter' s Plate Machine. — In this the rubber and 

 the points of the prime conductor are more widely separ- 

 ated ; and the prime conductor can therefore acquire a 

 higher level (or potential) of charge without discharge 

 over the glass to the rubber. The rubber can be insulated 

 or not, as required. A curious feature is an addition to the 

 Vol. XXXVII,— No. 949. 



prime conductor in the shape of a large ring of brass 

 inclosed in baked wood. This ring increases the 

 ' capacity ' of the prime conductor." 



The description of Winter's machine is not even sup- 

 plemented with a figure ; and we doubt if any student 

 reading the description will form the faintest conception 

 of the nature of the machine or of the " ring of brass 

 inclosed in baked wood." " Amalgamated silk," too, is a 

 shortened expression, which is, to say the least, as inap. 

 propriate as it is uncommon. These descriptions have 

 not been specially chosen for inadequacy. There are 

 numbers no more complete than these. 



Probably the chapter which will be found most satis- 

 factor by learners is the long and important Chapter X., 

 which deals with electro-static potential. This, with the 

 exception of the first two or three sections, is very com- 

 plete and well given. The subject is explained with 

 great clearness, and with abundant reference to numerical 

 calculation. 



The chapters on dynamo-electric machines and on 

 motors may also be considered fairly good for an element- 

 ary text-book. The learner will obtain in these chapters 

 a sufficient account of the principles of these machines, 

 given with satisfactory clearness. 



In his treatment of the subject of units, and particu- 

 larly of the electro-magnetic units, the author is singularly 

 unhappy. In an elementary book, or in any book on 

 this subject, whether elementary or advanced, it must be 

 considered a fundamental mistake to omit a full and clear 

 explanation of the foundation and derivation of the abso- 

 lute electro-magnetic unit of resistance ; and it is utterly 

 unsatisfactory to give, as a definition of the unit of re- 

 sistance, the remark merely that " Ohm's law defines the 

 unit of resistance as that through which unit electro- 

 motive force gives unit currer\t." It was not in this way 

 that the absolute unit of resistance was fixed upon, and 

 the original definition is certainly worthy of the attention 

 of the student. Taking the statement given above, how- 

 ever, and turning to "Ohm's law" for information, the 

 learner finds no statement of this law in words, but merely 

 the following : — 



" Ohm's law is that — 



C is proportional to -, 

 K. 



C = /& 



E„ 

 R" 



Such a statement as this might perhaps, if reproduced 

 for the benefit of an examiner, serve to conceal the 

 ignorance of the individual under examination, and might 

 leave the examiner so uncertain that he would be obliged, 

 though unwillingly, to award half marks to the answer ; 

 but to the student it can do no real good. 



There is but one other remark on the electro-magnetic 

 unit of resistance, and it is almost equally infelicitous with 

 what is quoted above. It is contained in a " note " on 

 " Determination of Units," and is to the effect that "re- 

 sistance can be measured by observation of the heat 

 evolved when a known current flows through the conductor 

 in question." A very slight acquaintance with possibilities 

 in experimenting would dispel any such idea. 



In connection with explanations regarding units we 

 meet here the customary sections on " Dimensions of 



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