Magnetic Circuit of Dynamo Machines. 503 



Then ft is known, so that if the lengths and cross sections of 

 the iron everywhere are known, the iron magnetic resistance 



2 — is known. Inserting this value of N as "N 7 in (7) we 

 a/j, 



find 47To . 



*'-&«^ 2(rf +l"fr 8 • • (11) 



a 2 ajj, 



and it will be found that this is a maximum for different 

 values of t when 



2* = M + 2 fc (12) 



a 2 a 2 afi 



That is, when the magnetic resistance of the space^ occupied 

 by the winding of the armature is equal to the resistance of 

 the rest of the magnetic circuit. 



When this is the case (11) reduces to 



W'=^f^|S 2 A 2 (13) 



That the ampere-turns on the field-magnet S 2 A 2 may really 

 produce the given induction 8 1 in the armature, it is necessary 

 that 



it a 2 



and if we insert this value in (13) we have of course (7) 

 again. 



It is now the custom in making dynamos to let 8 be nearly 

 constant in all parts of the circuit where the iron is the same, 



and to have - as nearly constant as possible, if different kinds of 



/* R 



iron are employed. If — is larger anywhere than its average 



(*> 

 value for the rest of the circuit, we say that the induction is 

 there "throttled," and throttling is only allowable in the 



B . 

 armature, if it is allowable anywhere. If — is nearly constant 



I P 

 everywhere in the iron, we may take — as the total iron re- 



sistance of the circuit, I being the average length of the lines 

 of magnetic induction in the iron of the whole circuit, and 

 then we may use 



— instead of ^ — 

 a x fi ^a/j, 



2L2 



