232 
PROFESSOR J. A. EWIRG AND MR. W. LOW ON THE 
in tlie magnet cores and pole-pieces cannot have exceeded 1400 when the magnetising 
current was at its strongest. The distribution of this over the conical pole-faces was 
not uniform. Close to the neck it reached the saturation value (of, say, 1680 or 
1700), being gathered there at the expense of outlying portions. This want of 
uniformity of 3 in the jDole-faces increases the magnetic force in the neck, but when a 
distribution of 3 is assigned it is easily taken account of in applying the formula of 
§ 15 . 
§ 23. Taking the experimental case (§ 7), in which the diameter of the neck was 
-^5 of the diameter to which the cones spread, we calculate that the magnetic force at 
the middle of the axis was probably about 22,500, and at other points of the axis it 
was less. 
Now, the measured value of the field in the air, close to the bobbin’s neck, was m 
this instance 25,620. To produce this force at the axis would require that the value 
of 3 in the pole-pieces should have been nothing less than 1690 all over, that is to 
say, it would require that the poles should have been saturated from axis to circum¬ 
ference—a quite impossible supposition. It is clear that the force in the air close to 
the neck was in this case distinctly greater than the mean force within the neck. 
The measured induction S within the neck was 43,500. If we accept 22,500 as 
the mean value of within the neck (remembering that while ^ increases from the 
axis to the circumference it diminishes from the middle towards the ends), the value of 
3 in the neck would be (43,500 — 22,500)/47r = 1670, which is, as nearly as may be 
judged, the same value as was produced by the application of magnetising forces of 
moderate strength. 
§ 24. These considerations establish a very strong presumption that the apparent 
decrease of 3 in the experiments, that is to say, the observed decrease in the quantity 
(33 — outside field)/47r under very strong forces is to be explained by the fact that 
the outside field was stronger than the field within the neck ; and that the true 
value of 3 is sensibly constant throughout the range of magnetic forces examined, 
namely, from about 4000 to 24,000 c.g.s. units. 
Further Experiments on Wrought Iron. 
§ 25. To put this matter further to the proof, we continued the experiments with 
another bobbin, also of Lowmoor wrought iron, the conical ends of which were shaped 
so as to produce a much more uniform field. The shape which would give the most 
uniform field was not chosen, for that would have imposed so low an upper limit on 
the strength of the field that the test would have been rather inconclusive. By way 
of compromise, a bobbin was turned of the shape and dimensions shown in fig. 8, with 
cones of semi-angle 45°, as a form which combined high concentrative power with 
a fair approximation to uniformity of field. The advantage, in respect of uniformity 
