The inverse of^ is related to the 'reluctance' R of the 

 magnetic material: 



R = /la '"> As ^decreases, R increases. 



As R increases in a material it shows a higher resistance 

 to the flow of magnetic flux (B) for a given Magneto 

 Motive Force H. This is an advantage for the LF sensing 

 section of the Magnograph as a dipole created by a local 

 fault will experience a very small 'short circuit' effect 

 by the surrounding wires . This allows detection of breaks 

 deep within a large rope. 



3.3.2 Permeability Changes in Magnetic Materials 



The magnetic behavior of steel in wire rope is affected 

 by factors other than the operating point of the magnetic 

 circuit used to test it. Temperature and tension change 

 the permeability of the steel and oxides associated with 

 corrosion can contribute to the total flux flowing within 

 a wire rope. The effects of temperature and tension on 

 permeability are very pronounced at low values of magnetic 

 flux density. 



At very high levels of magnetic flux density the effects 

 of temperature and tension are negligible. The contribu- 

 tion to the total flux within a rope under test due to 

 corrosion is also negligible when compared with the total 

 flux due to the steel alone. For these reasons, the 

 metallic area readings of the Magnograph do not require 

 'interpretation' to remove the effects of tension, temp- 

 erature or corrosion. 



A-14 



