3.2 The Magnetic Circuit 



The heart of the magnetic circuit is the rare earth 

 magnets at each end of the sensing head. As shown in 

 Fig. 3.2 the magnetic circuit is completed through the 

 flux bar on one side and through the pole piece and 

 rope under test on the other side. 



There are 4 sets of adapter tubes with various IDs 

 which fit against the pole pieces and which serve to 

 decrease the air gap between the pole piece and rope 

 under test. These are shown in Fig. 1.0. 



The magnetic circuit in the Magnograph brings the por- 

 tion of the rope between the pole pieces to a high level 

 of static magnetic saturation. In this state the mag- 

 netic flux density will have a uniform distribution 

 throughout the rope within the sensing head and will 

 assume a constant value. 



3.2.1 Metallic Area Measurement 



As the magnetic flux density (B) within the rope is at a 

 constant saturation value the total magnetic flux passing 

 through the rope and pole pieces is directly proportional 

 to the metallic cross-section of the rope. This flux is 

 what is measured by the LMA Hall sensors . 



When the metallic area of rope decreases or increases 

 along its length the LMA Hall sensors will show decreas- 

 ing or increasing voltage due to the change in metallic 

 cross-section. The voltages generated by the Hall 

 sensors are amplified and scaled in the electronic con- 

 trol section to produce outputs on the chart recorder 

 and magnetic tape recording. These outputs show changes 

 in the metallic area of the rope expressed as a percentage 

 of the nominal rope metallic area. 



3.2.2 Local Fault Measurements 



The magnetic flux induced into the rope under test will 

 be contained by the rope unless there are physical dis- 

 continuities within it. Physical discontinuities such 

 as broken or heavily nicked wires, pitting caused by 

 corrosion, or wire ends produce small dipoles, as shown 

 in Fig. 3.3. The resulting 'fringing field' of these 

 dipoles protrude outside the physical confines of the 

 rope . 



A-6 



