INTRODUCTORY 



3 



this end in view : it does not treat of coreless solenoids or magnetic 

 mechanisms with relatively long air gaps; because the air clear- 

 ance is always small in dynamo-electric machinery. 



In the method of design as followed in this book, an attempt is 

 made to base all arguments on scientific facts, and build up a 

 design in a logical manner from known fundamental principles. 

 This is admittedly different from the method followed by the 

 practical designer, who uses empirical formulas and "short 

 cuts," justified only by experience and practical knowledge. 

 It must not, however, be supposed that a commercial machine 

 can be designed without the aid of some rules and formulas 

 which have not been developed from first principles, for the 

 simple reason that the factors involved are either so numerous 

 or so abstruse that they cannot all be taken into account when 

 deriving the final formula or equation. In any case the con- 

 stants used in all formulas, even when developed on strictly 

 scientific lines, are invariably the result of observations made on 

 actual tests; and many of them, such as the coefficients of fric- 

 tion, magnetic reluctance, and eddy-current loss, are subject to 

 variation under conditions which it is difficult to determine. 

 The formulas used in design are therefore frequently empirical, 

 and they yield results that are often approximations only; but 

 an effort will be made to explain, whenever possible, the scientific 

 basis underlying all formulas used in this book. 



A perception of the fitness of a thing to fulfil a given purpose 

 and of the relative importance of the several factors entering 

 into a problem, is essential to the successful designer. This 

 quality, which may be referred to as engineering judgment, is 

 not easily taught; it grows with practice, and is strengthened by 

 the experience gained sometimes through repeated failures; 

 but it is necessary to success in engineering work, whether this 

 is of the nature of invention and designing, or the surmounting 

 of such obstacles and difficulties as will arise in every branch of 

 progressive engineering. All the conditions and governing factors 

 are not accurately known at the outset, and a good designer is 

 able to make a close estimate or a shrewd guess which, in nine 

 cases out of ten, will give him the required proportion or dimen- 

 sion; he will then apply tests based upon established scientific 

 principles in order to check his estimate, and so satisfy himself 

 that his machine will conform with the specified requirements. 



