INTRODUCTION 3 



found in the collection at the back of the book, as well as 

 in the separate chapters describing them. 



When any formula is given which uses approximations, 

 the limits of its accuracy should be clearly stated so that 

 one can tell at a glance whether the method is sufficiently 

 accurate for the purpose in hand, or whether a longer 

 method giving greater accuracy is desirable. This is espe- 

 cially necessary in the calculation of transmission lines, 

 because approximate formulas are quite permissible for 

 lines only a few miles long, but become very untrust- 

 worthy when the length is increased to one hundred miles 

 or more. For this reason, each table of formulas has its 

 percentage and range of accuracy printed in a prominent 

 position, so that the most suitable method for any case 

 may be instantly chosen. 



