Magnetic Induction in Straight Iron Rods, 479 



This method does not require specimens of any particular 

 dimensions or shape, but it is most convenient to use specimens 

 of round or square section, the length being less than thirty 

 times the section. 



It is well-known that when a specimen of iron of these 

 relative dimensions is placed in a magnetic field, the induction 

 varies greatly from the centre to the end. It is therefore 

 obvious that in measuring the hysteresis loss by the method 

 above referred to, each part of the specimen passes through a 

 different cycle. 



Since the hysteresis loss is expressed in relation to the 

 amplitude or maximum induction of the cycle, it is necessary 

 to know the amplitude of the cycle (termed Bi for reference) 

 through which the whole specimen would have to be taken to 

 give the same loss as that measured. If the amplitudes of 

 the cycles through which the different parts of the specimen 

 pass are known, the value of B x can be easily calculated. 



It has been shown by Steinmetz, and confirmed by others, 

 that the hysteresis loss can be expressed in relation to the 

 amplitude B of the cycle by the equation 



L = KB 16 , 

 L being the loss per cubic centimetre per cycle, and K a 

 constant depending on the quality of the iron. 



From this equation it follows that if L x be the loss measured 

 by the above method 



L 1 = KB 1 1 - 6 /s = KJ6-B 1 -^//, 



I being the length of the specimen, s the section, and dl the 

 differential of the length. 

 From the above equations 



Bx = ( J — — J l b = ^B^mean. ... (1) 



When the specimen is in the centre of the field it is obvious 

 that the induction in each part will be a maximum for that 

 part. If therefore the distribution of induction when in this 

 position is known, the value of B x can be calculated by 

 equation (1). 



To determine the distribution of induction in a given speci- 

 men requires some time and is somewhat tedious. It was 

 therefore decided to carry out a series of experiments with a 

 view of obtaining, if possible, the value of B x by a simple 

 and direct method. 



The experiments consisted mainly in determining the dis- 

 tribution of induction in specimens when placed in a uniform 

 field, 



2 L2 



