( 192 ) 
though it is rather slight. It proved useful again to let the vertex 
angle increase from the centre towards the edge, from about 56° to 
59°; instead of by a broken line, this transition can also take place 
by means of an arc of 3°, and corresponding — rather large — 
radius; the surface of revolution then becomes somewhat toroidal. 
A great number of field measurements with a very small test-coil 
were first made in order to gradually approximate towards the optimum 
form, to which, however, rather a “flat maximum” corresponds, 
which leaves the designer a certain margin. The final results are 
contained in the following table 
End planes: 
distance (mm. 
b mm. 
Field (kgs) 
50 (extr.) 
48.7 
45.; 
3.6 mm. 
Field (kgs) 
55 (extr.) 
52.2 (46) 
43.7 
These fields were obtained with 124 kiloampere-turns (with extra 
polar coils); the first limiting value has been linearly extrapolated 
and is of* some theoretical interest; for some experiments intervals 
below 0,5 mm. are already sufficient. I have placed between paren¬ 
theses the field values obtained by screwing the s4me polar pieces 
into the old ring-electro magnet, 108 kiloampere-turns (without extra 
polar coils) being used in this case. 
In fig. 2 a smaller type is represented in an erect position at l /i 
of the real size. This is essentially a simplified linear reduction to 
about 2 /, ; it is well-known that small electro-magnets are relatively 
much more efficient than larger ones, though it is more difficult to 
spare sufficient space for the ampere-turns required in the same 
linear ratio. The conic pole-tops with their bevilled edges as in 
fig. 4 — are cast in one piece with the cores themselves, and 
properly finished on the lathe. This apparatus weighs but 33 kg.; 
with 24 kiloampere-turns — only 7, of the above-mentioned value — 
the following approximate results were obtained: 
End planes: 6.7 mm. 3.6 mm. 
distance (mm.) Field (kgs) Field (kgs) 
0 36 (extr.) 40 (extr.) 
0,5 34 37 
1,0 
1,5 
30 
* .vish to express my great indebtedness to Prof. Kctaro Honda for 
his kind assistance in carrying out the necessary field measurements. 
