( 191 ) 
turns with 5(6) kilowatts, and the heat generated by the current 
amounts to 1,2 (1,45) kilocalories per second. 
With this apparatus the usual disk-poles and plane polar pieces 
are supplied to obtain extensive uniform fields. For stronger fields 
the diameter is reduced in the first place from 93 to 43 mm. by 
means of “pole-shoes” P of special broken truncated shape (fig. 3 
real size). They consist of annealed Swedish laminated iron ; their 
most efficient form was again empirically determined and hardly 
deviates from the shape used until now 1 ). Interpolar pieces of 
different kinds may then be placed between the two smaller circular 
end-planes, as required for experiments. 
The comparatively small diameter of 43 mm. makes it easy and 
desirable for every laboratory itself to have such pole-tops Q turned 
out of Swedish iron, as are best adapted to any special purpose. 
Their contribution to the total field amounts to 80 to 90 percent, 
and thus far exceeds that of the pole-shoes, which as a rule only con¬ 
tribute 10 to 20 percent; the influence of the conic surface-elements 
increases very much as they lie nearer the extremities, where they 
should be carefully shaped and centred; this purpose is effected by 
special centred distance-pieces of unmagnetic material. Thus in course 
of time polar pieces were made for cryo- and pyromagnetic experi¬ 
ments, for micro-magnetic and magneto-optical investigation (Faraday, 
Kerr and ZEEMAN-effect), for the torsion-balance and magneto-hydro¬ 
static method and that of the isthmus for magnetic stress determina¬ 
tions. A fuller description of these various arrangements would claim 
more space than we can afford to give it here; it will appear else¬ 
where with a discussion of the general principles for the design of 
such intrapolar apparatus, as are adapted to various purposes. 
In the conic borings (see fig. 3) split cores or solid filling-plugs 
may be introduced according to the nature of the experiments. With 
the pole-tops reproduced in fig. 3, whose end-planes have a diameter 
of 3,6 mm. the strongest field was empirically obtained through 
calculation, measurement and mechanical shaping proceeding simulta¬ 
neously. On the other hand the dotted lines represent the well-known 
theoretical optimum form consisting of a cone with a vertex semi¬ 
angle of 54°44', and part of the “isthmus”. The theory must start 
from the assumption of absolute saturation in the direction of the 
axis, which can never really be fulfilled; hence the actual deviation, 
i) Since 1901 slightly rounded pole-shoes were supplied with large half-ring 
electro magnets. No pole-shoes were required for the smaller ones, and it proved 
useful, on the contrary, to provide their pole-tops with a bevilled edge, which 
* peripherically increases the effective conic surface, (see fig. 4). 
