DESIGN OF CHOKING COILS. 87 



Now, let A be the area in square centimetres of the cross 

 section of the iron core, and B the maximum induction in the core, 

 so that 



F = AB 



and 



NF = NAB 



The maximum induction B may be fixed arbitrarily, thus 

 giving the product NA. It is not necessary to keep the value of 

 B so low as in transformer cores, because a choking coil is only in 

 use while the accompanying apparatus is working, and hysteresis 

 and eddy- current losses are not of such great importance as if, as 

 is often the case in transformers, the current passed through 

 it constantly, and the iron loss was always going on. 



The determination of N and A separately is a matter of design, 

 and is mainly a question of experience. 



WQ will complete the calculation in a specific case. 



A choking coil is required for use with a 40-volt 5 -ampere arc 

 lamp on a 100-volt 50 frequency supply : give the necessary 

 details of the coil. 



The voltage v required between the terminals of the coil is 



v = \/100 2 - 40 2 



= 91*6 volts, nearly 

 therefore 



= 91-6 



10 8 \/2 

 whence 



NF = 41-23 x 10 6 

 Suppose that 



B = 14,000 

 then 



NA = 2943 



Now, if the iron core forms a closed circuit, so as to have a 

 small reluctance, and if we take 



N= 100 

 we have 



A = 2 9 '43 square centimetres. 



Since B = 14,000, the permeability is 823. 



