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when such a core is used, and the arm- 

 ature conductor must therefore be made 

 in the form of a very narrow strip ; 

 so narrow, in some cases, that two or 

 more conductors must be placed in par- 

 allel to obtain the required cross-section- 

 al area. The author is unable to suggest 

 an entirely satisfactory explanation for 

 the effect of the iron core in reducing 

 eddy currents in the copper, and merely 

 mentions the fact as deserving attention. 

 In direct current machines he has ob- 

 served a somewhat similar phenomenon. 

 When the pole-pieces are provided with 

 extensions, the heating of the armature 

 bars is much less than when these exten- 

 sions are absent, and consequently the 

 trouble of heating can to a certain de- 

 gree be overcome by the simple expe- 

 dient of fitting so-called " horns " to the 

 pole-pieces. Another cause of the gen- 

 eration of heat, in machines of all types, 

 is the change of magnetization in the 

 iron parts. This can be minimized by 

 lamination, but never entirely avoided. 

 In direct-current machines the heating 



