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BELL SYSTEM TECHNICAL JOURNAL 



winding giving a satisfactorily low resistance even in a coil only a 

 fraction of the size of the equivalent air core coil. An adaptation of 

 the new magnetic material, permalloy, has made this type of inductance 

 standard possible.^ Their cores consist of finely laminated, high 



F"ig. 16 — Coil and dial switch asscmbh' of typical inductance stanflard decade 



specific resistance permalloy punchings, carefully annealed and as- 

 sembled to form a toroidal structure whose effective permeability is 

 about forty. On this is wound a sectionalized winding of insulated 

 stranded conductor, the individual strands also being insulated from 

 each other. The wound coils, after adjustment to the value desired, 

 are sealed with moisture-proof compounds in phenol fiber cases. Fig. 

 16 shows an assembly of the four coils and switch which comprise one 

 decade of the standard. In Table III are given data for typical coils 

 illustrating their performance in respect to the above points. 



The adjustable, non-inductive resistance is a commercial dial 

 resistance box to which a shield has been added. It has five dials 

 providing a range of 1000 ohms in steps of 0.01 ohm. Its shield is 

 grounded in use, the resistance itself being connected usually between 

 the C corner of the bridge and the inductance standard but in the 

 case of an unknown impedance having a lower resistance than the 

 standard from the C corner to the coil under test. 



^ H. D; Arnold and G. W. Elmen, Franklin Institute Journal, 195, 1923. 



