SHUDDEMAGEN. — DEMAGNETIZING FACTORS FOR IRON RODS. 243 



Dimensions of Iron Rod: Diameter = 5 mms. Length = 20 cms., so 

 that 111 = 40. 



Secondary Coil: 480 turns of fine wire. Length = 3 cms. Resist- 

 ance = 19.42 ohms. 



We therefore neglect the leakage of induction through the secondary 

 coil. If we have no extra resistance in the galvanometer circuit the 

 formula gives for the method of reversals : 



B 



100 -i? 



42-31 



= 2400. 



T /S'-2 7r(0.25)-480 0.00695 TT- 0.60 



This shows that we need no extra resistance for the secondary circuit. 

 Suppose we magnetize in a solenoid 31 cms. long and wound with 

 5 layers of wire, 113 turns in each layer. Then we have 



W = ■ (No. of amperes) = 22.9 (amperes). 



10-31 



We get the following observations: 



giving the calculated results : 



