A REVIEW OF NEW MAGNETIC PHENOMENA 1159 



what below that indicated by /i in Fig. 3, depends upon the particular 

 ferrite. The range of applicability of equation (1), therefore, varies. 

 Regardless of frequency, the eddy current and hysteresis terms in equa- 

 tion (1) are valid. However, additional terms are required to cover other 

 losses which become quite high and in comparison with which the 

 "residual" term in equation 1 may be negligible. 



Frequently, the separation of losses indicated in equation (1) is not 

 called for. The practice then is to lump all losses together and express 

 them in terms of the material Q, which may be a function of frequency 

 and fiux density. Here Q is the ratio of the reactance of the winding on a 

 toroidal ring to the core loss expressed as a series resistance, 



The product juQ is convenient in describing magnetic characteristics. 

 Sometimes air gaps are introduced in ferrite magnetic cores in order to 

 provide higher coil Q's or greater stability of ac permeability with super- 

 posed dc magnetizing force. It can be shown that the product juQ re- 

 mains constant even though the core is divided by one or more air gaps. 

 Fig. 2 shows curves of ^lQ versus frequency for various ferrites and also 

 for certain other materials. An extensive discussion of methods of meas- 

 urement and of results of measurements at low frequencies is given in a 

 recent paper by Owens.^ 



It should be pointed out that eddy currents in ferrites are so small 

 that solid shapes are usually used whenever ferrites are applicable. This 

 is a considerable advantage, eliminating the necessity for thin tapes or 

 insulated fine particles which are required in many applications of me- 

 tallic cores. However, so far, no ferrite has been developed which has 

 permeability nearly as great as that of some of the permalloys. 



V. CHARACTERISTICS ABOVE 1 MC 



So far, our discussion has covered the frequency range in which mag- 

 netic materials have traditionally found many important uses and in 

 which the ferrites have properties generally like other magnetic ma- 

 terials, differing from them only in degree. We now come to the fre- 

 quency range in which new magnetic phenomena have recently been 

 observed — a range above that in which magnetic materials have here- 

 tofore been generally applicable. 



In discussing the higher frequency characteristics, it is desirable to 

 introduce a somewhat different set of parameters by which the properties 



