CHAPTER XIX 

 INDUCED MAGNETISATION 



Magnetic induction Induction and intensity in air Lines and tubes 

 of induction in air Induction within a magnetised body Hydraulic 

 illustration Equations expressing continuity of potential and continuity 

 of induction tubes Representation of induced magnetisation by an 

 imaginary distribution of magnetic poles acting according to the inverse 

 square law Magnetic susceptibility Imagined investigation of B and 

 H within iron Permeability and the molecular theory Induction in a 

 sphere of uniform permeability /* placed in a field in air uniform before its 

 introduction Long straight wire placed in a field in air uniform before 

 its introduction Circular iron wire in a circular field Energy per 

 cubic centimetre in a magnetised body with constant permeability The 

 energy supplied during a cycle when the permeability varies Calculation 

 of induced magnetisation is only practicable when the permeability is 

 constant Diamagnetic bodies Magnetic induction in a body when the 

 surrounding medium has permeability differing from unity. 



Magnetic induction. We have seen that when a mag- 

 netisable body, such as a piece of iron or steel, is introduced into 

 a magnetic field it becomes a magnet by induction. We shall 

 investigate in this chapter methods of representing this induced 

 magnetisation. 



The magnetic force on a pole placed at any point in the air 

 surrounding the magnetised body may be calculated as if it were 

 due to the direct action of polarity scattered over the surface 

 or through the volume of the body, each element of the polarity 

 acting directly on the pole with intensity inversely as the square 

 of the distance. But it must be remembered that this is a mere 

 working representation, adopted for the purpose of calculation. 

 The magnetic condition must be regarded as existing at every 

 point in the field, whether a test pole is placed there to reveal it or 

 not. The force on a pole in the field is due to the altered the 

 magnetic condition of the medium immediately round it. 



Faraday, who introduced the idea of action through and by the 

 medium into magnetism as well as into electricity, introduced also 

 the use of lines of force, or, as we shall here term them, lines of 

 induction, to describe the condition of the field, and he showed that 

 these lines gave not only the direction of the intensity but also 

 that by their crowding or sparseness they gave its magnitude. He 

 was led by his idea of an altered condition of the medium to the 



229 



