2 PRINCIPLES OF ELECTRICAL DESIGN 



Having clearly realized the general shape and distribution 

 of the magnetic field surrounding a conductor or linked with a 

 coil of wire carrying an electric current, the next step is to cal- 

 culate with sufficient accuracy for practical purposes the quantity 

 of magnetic flux produced by a given current; or the e.m.f. 

 developed by the cutting of a known magnetic field. This leads 

 to the consideration of units of measurement. 



The practical units of the C.G.S. system will be used so far as 

 possible; but since engineers of English-speaking countries still 

 prefer the foot and inch for the measurement of length, there 

 must necessarily be a certain amount of conversion from centi- 

 meter to inch units, and vice versa. This may, at first sight, 

 appear objectionable; but, in the opinion of the writer, there is 

 something to be gained by having to transform results from one 

 system of units to another. The process helps to counteract 

 the tendency of mathematically trained minds to lay hold of 

 symbols and formulas and treat them as realities, instead of 

 striving always to visualize the physical (or natural) reality 

 which these symbols stand for. The same may be said of such 

 alphabetical letters as are in general use to denote certain physical 

 quantities or coefficients; as /* for permeability, and L for the 

 coefficient of self-induction. Familiarity with these symbols 

 tends to obscure the physical meaning of the things they stand 

 for; and although uniformity in the use of symbols in technical 

 literature cannot be otherwise than advantageous, 1 the use of 

 unconventional symbols involves their correct definition, and 

 for this reason their occasional appearance in writings that are 

 professedly of an instructional nature should not be condemned. 

 This point is made here to emphasize the writer's conviction 

 that the student should endeavor to regard symbols and mathe- 

 matical analysis as convenient means to attain a desired end; 

 and that he should cultivate the habit of forming a concept or 

 mental image of the physical factors involved in every problem, 

 even during the intermediate processes of a calculation, if this 

 can be done. 



By way of illustrating the application of fundamental magnetic 

 principles, the design of electromagnets will be taken up before 

 considering the magnetic field of dynamos. This preliminary 

 study should be very helpful in paving the way to the main 

 subject; and the chapter on magnet design has been written with 



1 A list of the symbols used will be found at the beginning of this book. 



