240 MAGNETISM 



to PQ, then if 2 is the angle which the axis of the tube makes 

 with the normal to PQ, 



MN = PQcos0 2 



and PQ x a- = MN X I = PQ X Icos0, 

 or a = I cos 2 . 



I is termed the intensity of magnetisation. It must not be 

 confused with H, the field intensity, or the force per unit pole. 

 Since I x MN is constant along the tube and since B X MN = 

 /xH X MN is also constant, I is proportional to/xH, or to H since 

 we are taking fj. as constant. 



Let us put I = /cH. 



K is termed the magnetic susceptibility of the iron, or, sometimes, 

 the coefficient of magnetisation. 

 Let us take MN close to PQ : 



We have <r = I cos 8 2 = *H cos 2 . 



But also a- = ^-j ^ It O 



4-7T 





and fui = 1 -f 



Imagined experimental investigation of B and H 

 within iron. Lord Kelvin imagined a method of investigating 

 the values of B and H within iron which, though experimentally 

 unrealisable, is a valuable aid to understanding the difference be- 

 tween the two quantities, induction and intensity of field. Let us 

 suppose that a long and exceedingly narrow tunnel is made along 

 a line of induction in the iron. If narrow enough it will not 

 disturb the magnetisation in the iron round it except very close to 

 the ends, and at a distance from the ends the intensity within the 

 tunnel will be the same as in the iron close to its sides, or as in 

 the iron before the tunnel was made. We may regard this as a 

 consequence of the condition of continuity of potential, or if we do 

 not wish to assume that condition we must regard this as a defini- 

 tion of H within the iron. Since the tunnel is free from iron wo 

 can imagine H to be measured within it by means of the vibration 

 of a minute needle of known moment. Since the needle is in 

 iron-free space we can calculate the intensity of the field in which 



