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PROFESSOR J. A. EWINO AE'D MR. W. LOW ON THE 
impracticable to produce quite so great a force as this on account of the difficulty of 
saturating the magnet poles. 
§ 17. With the cones which give the most uniform field, for which sin 6 = and 
cos 6 = v/f j file value of F is only 
8-965 3 logio 
which becomes 
15240 logio^^ 
in the event of the pole-pieces being of soft wrought iron and saturated. 
§ 18. The curve, fig. 5, has been drawn to show how the force at the vertex varies 
when the angle of the cone is altered. 
Fig. 5. 
The base of the cone being represented by AB, any ordinate PM gives the force 
when the vertex is at M. In the figure, A MB represents the cone of maximum 
concentrative power, and ANB represents the cone giving a uniform field in the 
neighbourhood of the axis, Q being the point of inflection in the curve. 
§ 19. In hgs. 6 and 7 the same two cases are further illustrated by curves which 
show the sum of the forces due to two equal and opposite rings (situated on cones 
witli a common vertex) at points along the axis. 
By summing up the effects of such pairs, for the whole cone, we may judge how 
nearly the force is uniform from end to end of the neck of the magnetised bobbin. 
In a bobbin with cones of semi-angle 14' the field is sensibly uniform from end to 
end of the neck, except close to the ends, where it is slightly reduced, and (§ 14) this 
longitudinal uniformity implies transverse uniformity. 
§ 20. When the semi-vertical angle of the cones is greater than 39° 14', the force at 
points on the axis has a maximum at the common vertex, and, since d^FJdx" and 
d^Fjdy^ have opposite signs (§ 14), the field is stronger at places near the axis than 
