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Physics. — “A theory of polar armatures.” By H. pu Bors. (Com- 
munication from the Bosscha-Laboratory). 
A well-known partial theory for truncated cones was given by 
Strran and applied to the isthmus-method by Sir Arrrep Ewing. As 
a first approximation the magnetisation of the poles is everywhere 
Fig. 1. 
assumed parallel to the z-axis (Fig. 1) and thus polar elements have 
to be dealt with on the terminal surfaces only. 
Now the magnetic field due to coils of various shapes has been 
thoroughly investigated in every detail by various authors, whereas 
that produced by ferromagnetic pole-pieces is only known for parti- 
cular points in a few special cases. I believe it is now useful to 
develop a more general and complete theory for arbitrary points 
in the field, regard being also paid to protruding frontal surfaces, 
such as I have been using since 1889 (see fig. 1). 
Considering the increasing introduction of prismatic pole-pieces, 
e.g. for string-galvanometers and other applications, I have also 
calculated equations for these, generally exhibiting a formal analogy 
with the conic formulae. Instead of a meridian section, Fig. 1 in 
this case represents a normal section, the generatrices being directed 
normally to the plane of figure and parallel to the z-axis. 
For the determination of attraction or repulsion the first derivatives 
of the field with respect to the coordinates have to be considered ; 
e.g. for gradient-methods in measuring weak para- or diamagnetic 
susceptibilities and also for extraction-magnets, such as those used 
in ophtalmologie surgery and in ore-separators. 
Besides the intensity of the field its topography, especially its 
more or less uniform distribution appears more and more important 
in quantitative work and ought to be investigated. Here the second 
derivatives of the field also come in. 
The following equations may occasionally serve as well for certain 
