M. JAMIN'S FORMULAE. 397 



For long bars, this latter condition reduces to 

 A = 27T0/&F 



and shows that the constant A is proportional to the perimeter. 



423. Green's formula corresponds to the case of a cylinder 

 placed in a uniform field parallel to the axis, and for which the 

 coefficient of magnetisation is constant Professor Rowland has 

 pointed out .the analogy of this formula with that which expresses 

 the lateral flow from a pile of the same form placed in a conducting 

 medium (268). Let us suppose that the flow of magnetic induction 

 is propagated like the flow of electricity; if we retain the same 

 meanings for the quantities /o, /a', and R x , and if we replace the 

 quantity by the force F of the field, and if we call Q the flow 

 of magnetic induction across a section of the bar at the distance x 

 from the centre, we have, for the flow in the interior, 



(8) 



i - 



and, for the lateral flow, 



These formulae also apply to the case in which the magnet is 

 solenoidal, bounded by a channel surface closed upon itself, and 

 the magnetisation of which is everywhere perpendicular to the right 



p 

 section. We have, in that case, Rj0, Q = , and the flow of 



P 



lateral induction is zero. If F 1 is the magnetic induction, s the 

 section of the bar, and /* the coefficient of permeability, we have, 

 further, 



and therefore /* = . 

 ps 



424. All experiments go to prove that magnetisation tends 



