MAONETISATIOK OF IRON AND OTHER MAGNETIC METALS. 
231 
on the axis itself. In a bobbin with a narrow neck this may have the effect of 
making the field in the closely surrounding air space greater tlian the mean field 
within the neck. 
§ 21. We may now apply the above conclusions to elucidate the experiments which 
have been described. The form of bobbin used in them had been chosen, without 
reference to theory, as one likely to give a strong concentration of magnetic induction, 
and it chanced to come very near the best form for this purpose. The cones had a 
semi-angle of 60°, and their vertices were nearly coincident (overlapping very slightly, 
see fig. 2). 
Applying the formula of § 1.5, we have, for 9 = 60°, 
F = 10-85 3 logio ^ ’ 
which is only two and a half per cent, short of the force attainable by using cones of 
maximum concentrative power. Moreover, it must be borne in mind that in actual 
use of the isthmus method the strongest induction will be reached when the semi-angle 
is rather greater than 54° 44', for 3 is itself a function of the angle, decreasing when 
the angle is decreased, on account of the augmented “ resistance ” of the whole 
magnetic circuit. For this reason we probably obtained as much concentration wdth 
cones of 60° as we should have obtained with cones of 54° 44'. Further, in the last 
experiment (§ 8), tlie neck of the bobbin had been turned down to the smallest size 
we found it practicable to work with. It is clear, therefore, that no materially higher 
value of SS than the value already obtained was possible with the apparatus at our 
disposal. 
§ 22. From the relation of the line integfral of maometic force to the length of iron 
and mean length of air in the magnetic circuit, we estimate that the mean value of 3 
