MAGNETISATION OP IRON AND OTHER MAGNETIC METAI.S. 
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brass tubes turned to slip one on another, wltli flauges at the ends to preserve a 
definite clearance between them and to keep them concentric. The innermost coil 
had a mean diameter of 3’2 mm., and the outermost a mean diameter of 7'8 cm. The 
annular space between them, 2‘3 mm. thick, was in this way divided into three parts, 
in each of which the field was measured. It was found that in these three parts the 
field decreased progressively with increase of distance from the axis. Thus, in one 
instance, the field fell from 19,000 in the innermost ring to 17,300 in the outermost 
ring. It is unnecessary to describe at length these experiments, which were very 
laborious, and which did not throw any light on the important question of how closely 
the force within the metal approximated to the force in the air close to the surface of 
the neck. Moreover, the form of the built-up bobbin used in this case was, as we 
afterwards recognised, such as to give a much more uniform field than the bobbin 
formerly used. 
Concentration of Magnetic Force by Conical Pole-faces. 
§ 11. The magnetic force in the space between the pole-pieces is made up of two 
parts: (1) the electromagnetic force directly produced by the current in the field- 
magnet coils ; and (2) the force due to free magnetism distributed for the most part 
over the pole-faces. The first of these was a comparatively small part (less than one- 
fiftieth) of the whole, and its value must have been sensibly uniform at such small 
distances from the axis as those with which we are now concerned. In considering 
the uniformity of the field we need only deal with the force produced by free magnetism 
distributed over the opposing surfaces of the poles. 
Fig, 1. 
§ 12. The free magnetism on each pole-face may be treated as made up of a series 
of co-axial circular rings in planes normal to the axis of magnetisation. Calling M the 
whole free magnetism of one of these rings (fig. 4) and r its radius, the force F due to 
it at a point in the axis at a distance x from the plane of the ring is where 
^ = C {F + x”). This force will be a maximum when clYjdx = 0, or 
1 3a;- 
2 G 2 
