HARMONIC PRODUCTION IN MAGNETIC MATERIALS 787 



in which the bracketed expression coincides with the variable factor 

 in equation (45). Hence when Li is constant the choice of core 

 material follows the rules laid down for inductance coils. When 

 the inductance is not constant, however, the factor becomes pro- 

 portional to 



. dao2 .^^. 



with the understanding that the turns ratio, resistances and frequency- 

 are fixed. Thus the secondary harmonic current with a given material 

 is reduced by increasing the turns and core area and reducing the 

 diameter. As far as core materials go, we have for the significant ratio 

 aoa/M^ the values: 



Material , 002/1^^ 



Silicon Steel 130 X 10"^ 



Permalloy Dust '« 2.4 X 10"^ 



B Dust 5.3 X 10-4 



C Dust 4.3 X 10-4 



Perminvar i^ 0.05 X 10-^ 



The great superiority of perminvar indicated above is restricted of 

 course to fields of the order of less than 0.1 gilbert/cm. above which 

 it tends to become smaller; at a field of 0.7 for example the above 

 factor for perminvar would be multiplied by the factor three. Perm- 

 alloy dust is observed to be approximately twice as good as the iron 

 dust cores. 



A very important question to answer with regard to transformer 

 cores is this — what benefit can be gained as to harmonic production by 

 inserting air-gaps, or by diluting the core material, leaving everything 

 else unchanged. This is evidently to be answered by an investigation 

 of the ratios ao2/M^ and a'o2/M'^ the primes referring as before to the 

 diluted material. These relations are given by Equation (42) in 

 which fjL and aio are interchangeable: 



-f-y ^10 ) = m'j 



flio = flio / (1+7 ^10 j 

 = ^02/ (1+7 «io 



ao2 = ao2 ( 1 +7 ai 

 These permit us to evaluate o'o2/m'^: 



1 j_^ y 



n ' r, 1 1 +7^10 



" Laboratory specimens. 



