METALLIC MATERIALS IN THE TELEPHONE SYSTEM 149 



section when hot, but when cold it is exceedingly brittle and can be 

 pulverized to fine powder. The manganese content of the alloy must 

 also be controlled since it has an effect opposite to that of sulphur. 

 The iron-cobalt system yields a useful magnetic material, the one 



Fig. 6 — Twenty roll cold-reduction mill for producing thin sheet 

 materials for experimental studies. 



containing approximately equal percentages of iron and cobalt. This 

 alloy, called perniendur, is characterized by high permeability at high 

 flux densities and by a high reversible permeability when subjected to 

 superposed direct current magnetizing forces. The binary alloy can- 



