1888.] Magnetisation of Fron, fyc., in very strong Fields. 41 



The induction was measured by means of a coil consisting of a 

 single layer of very fine wire wound upon the central neck of the 

 bobbin. Outside of this coil, at a definite distance from it, a second 

 coil was wound, and the magnetic force was determined in the annular 

 space between the two. In a paper communicated to the Manchester 

 meeting of the British Association, the authors showed that if the 

 force so measured could be proved to have the same value as the 

 magnetic force within the metal neck itself, it would follow that 

 the intensity of magnetism |j had begun to diminish under the 

 action of excessively strong fields, in the manner which Maxwell's 

 extension of the Weber- Ampere theory of molecular magnets antici- 

 pates. In the present paper the authors discuss at some length the 

 question of how far the magnetic force within the metal is fairly 

 measurable by the magnetic force in the ring of surrounding air, and 

 they show that with the form of cones originally used the force 

 within the metal must have been less than the force outside, by an 

 amount probably sufficient to explain the apparent decrease of J. 

 The form of cone suited to give a uniform field of force with sensibly 

 the same value in the metal neck and round it is investigated ; and 

 experiments are described in which the condition necessary for a 

 uniform field was satisfied. The results of these experiments are 

 conclusive in showing that no considerable change takes place in the 

 value of | (in wrought iron) when the magnetic force is varied from 

 about 2000 to 20000 c.g.s. units. Throughout this range of force, the 

 intensity of magnetism has a sensibly constant value of about 1700 c.g.s. 

 units, which is to be accepted as the saturation value for wrought 

 iron. The term saturation may be properly applied in speaking of 

 the intensity of magnetism, but there appears to be no limit to the 

 degree to which the magnetic induction may be raised. 



To produce the greatest concentration of force upon the central 

 neck, the converging pole faces should have the form of cones, with 

 a common vertex in the middle of the neck, and with a semi- vertical 

 angle of 54 44'. This form, however, does not give a uniform field 

 in the neighbourhood of the vertex. To secure that, the condition is 

 that d?~F[dx\ d 2 F/cfy 2 , and d 2 F/Wz 2 shall vanish, F being the magnetic 

 force at the vertex, which is due mainly to the free magnetism 

 distributed over the pole faces. The condition for a uniform field is 

 satisfied when the cones have a semi- vertical angle of 39 14'. When 

 this form is given to the cones, the magnetic force in the air im- 

 mediately surrounding the central neck may be taken as sensibly 

 equal to the force within the neck, and it therefore becomes practic- 

 able to measure the relation of the induction to the force producing 

 it, that is to say, the magnetic permeability. 



The greatest attainable concentration may be calculated by assum- 

 ing the pole faces to be saturated, when the cones are such as to 



