REVIEWS 347 



with each of these points it is possible to obtain certain numerical measures by 



what is called the " Method of Statistics." These measures give some idea of the 



value to be attributed to the figures, and without these measures no argument 



based on them can be of any value. Dr. Brend has not attempted to give such 



measures. He has shown that the figures are unreliable and incomplete, and 



gives useful suggestions for improvement which might well be considered by 



those occupied in recording. 



W. Stott. 



ENGINEERING 



Specification and Design of Electrical Machinery. By Prof. Miles Walker, 

 M.I.E.E., etc. [Pp. xix + 648, with illustrations.] (London : Longmans, 

 Green, 191 5. Price 32J. net.) 

 THE chief interest of this book to the non-technical person lies in the evidence it 

 gives of the progress that has been made in systematising the design of all kinds 

 of electromagnetic machinery. The electrical designer, though he has been the 

 last to come into the field, has always been the most scientific of all designing 

 engineers. The criticism that has been levelled at him by engineers of an older 

 school is that he is too scientific and that he does not pay enough attention to 

 the practical conditions under which the machinery he designs has to operate. 

 Though this criticism was possibly a just one as regards many designs of twenty 

 years ago it is not a just one to-day, and this book is sufficient evidence that the 

 designer of electrical machinery is now as alive to practical difficulties as any 

 other member of his profession. 



The early chapters of the book deal with the general question of electrical design, 

 and Prof. Walker follows the method devised originally by Mr. B. G. Lamne in all 

 the machines with which this book deals. The method is one of great interest, 

 and it is thoroughly scientific because it is based on what Dr. Thompson has 

 called the " specific utilisation of material." 



The essential things in any electromagnetic machine are, firstly, the magnetic 

 field in which the conductors which produce the electrical energy move, and 

 secondly, the conductors which carry the current. The strength of the magnetic 

 field depends on the density of magnetic flux that can be used in the iron ; 

 this flux passes through the iron circuit of the machine and across the air gap 

 to the copper-carrying part of the machine which supports the conductors. The 

 total flux passing across this air gap backwards and forwards is called the mag- 

 netic loading of the machine, and is evidently a quality depending principally on 

 the qualities of the material of which the frame of the machine is built up and on 

 its size. The power which the machine will develop, if it is a dynamo, depends 

 on the voltage which the machine produces and on the current which it will carry. 

 The voltage produced depends on the number of conductors which are cutting the 

 magnetic flux, and the current which the machine gives will depend ultimately on 

 the copper section of the conductors. The second factor in the design is thus 

 arrived at, i.e. the current loading, which is the product of the current per con- 

 ductor and the number of conductors carrying that current. The design of any 

 machine is thus reduced to a consideration of these two factors, and the ultimate 

 dimensions are determined by the limits of magnetic loading and current loading 

 that are possible. 



The first half of the book deals with the subject of design from this point of 

 view. It deals with the magnetic qualities of the materials used in dynamo manu- 

 facture, and with the practical limits of flux density which can be employed. It 



