86 TREATISE ON ALTERNATING CURRENTS. 



and the self-induction L of the choking coil is given by 



L = -.K/F 2 - ! 2 cos 2 - v, sin 9} . . . (4) 

 jP* 



If 6 = 0, equation (4) reduces to equation (2), as should be 

 the case, since that is the condition that the apparatus should be 

 non-inductive. 



As a rule, the value of 6 will be known for any particular class 

 of apparatus, such as induction motors, so that V, v\, and 9 are all 

 given, and L becomes determinate. 



DESIGN OF CHOKING COILS. 



58. Having determined the value of L for the choking coil 

 required, we proceed to indicate the principles upon which its 

 design and construction may be carried out. 



Let N be the number of turns of wire on the coil, and F sin pt 

 the flux of magnetic lines through the iron core at time t. 



Then the E.M.F. produced in the coil by the variation of this 

 flux is given by 



= pNF cos pt 



The R.M.S. value of e is therefore 



pNF 



or, reduced to volts, we have 



therefore 



pNF _ u 



10 V2 ~ P 

 or 



NF = W 8 \/2Li (6) 



Thus the product of the number of turns of wire, and the 

 maximum magnetic flux is given by equation (6). 



