CHAP. II] MAGNETIC CIRCUIT WITH IRON 31 



Prob. 18. What is the length of the yoke in Fig. 6 if the exciting cur- 

 rent increases 12 times when the flux density in the air-gap increases from 

 1 to 7 kl/sq. cm.? Hint: If H t and H 7 are the known magnetic intensi- 

 ties in the yoke, corresponding to the two given densities, and x is the 

 unknown length of the yoke, we have, using the values obtained in the 

 solution of problem 15: (// l x+416)12 = // J x-f 3090. 



Ans. About 1.2 m. 



