MAGNETISATION OP IRON AND OTHER MAGNETIC METALS. 
233 
of field, which this bobbin had over the one formerly used may be judged from 
figs. 9 and 10, which show the longitudinal variation of force due to a pair of rings in 
the two cases. The length and diameter of the neck were 3’42 mm. The outside 
field and the induction were measured as before, and it was found that they were 
decidedly less than in the former instance, chiefly, of course, because of the greater 
mean thickness of air space now present between the magnet poles, which reduced the 
Fig 8. 
mean value of 3 in them. But what is important to our present purpose is to note 
that now, owing to the greater uniformity of the field, the quantity (iB — outside 
field)/47r undergoes no progressive diminution as the force rises. Table IV. gives the 
results. They confirm the conclusion which was provisionally stated in § 24. Here 
we may accept the strength of the outside field as closely approximating to the mean 
force within the neck, so that the first column in the table might have been styled 
the second last column 3; and the last column /r. 
MDCCCLXXXIX.—A. 2 H 
