( 190 ) 
rests on a broad ball frame with a divided circle, so that the appa¬ 
ratus may easily revolve round a vertical axis; it can also be fixed 
by means of 3 set-screws. The adjusting screws on the left-hand 
side are useful for the vertical position of the magnetic axis occasio¬ 
nally required (see fig. 2); the hand-wheel allows gradual displace¬ 
ment of the left-hand core. In practice it is extremely useful and 
convenient to divide the magnetic circuit by the upper horizontal plane 
of the base part of the apparatus, so that each of the two cores 
may slide as well as revolve on it. Each core corresponds to about 
one third of a toroid and its lower end terminates in a cast flange. 
Its vertical circular end-planes have a diameter of 93 mm.; towards 
the bottom this gradually increases. 
The investigation of the Faraday and Zeeman effects carried out 
in this laboratory enforced the condition of great intensity of light, 
which may be fulfilled by concentration of the beam of rays in the 
interpolar space, so that it may form a strongly diverging double 
cone of light. Evidently the borings in the iron must correspond 
with this, from which the further condition of a short path of the 
rays immediately follows, as the width of the boring is naturally 
limited. Conical holes were bored 1 : 5 of a length of only 15 cm.; 
one of the iron filling-plugs to be used in non-optical experiments 
is represented in fig. 1; the length of the path of the rays within 
the iron is restricted to these borings themselves by the peculiar 
design of angular flanges, which appears sufficiently clearly in the 
figure. These flanges are provided with regularly spaced nuts, to 
which optical and other appliances may be screwed, so as to lie 
just at the mouth of the boring. The polar ends of the cores are 
surrounded by a thin copper tube for water circulation; 50 litres 
of water per hour works sufficiently well; moreover an inner circu¬ 
lation may be arranged within the borings; besides the pole-shoes 
may be provided with external cooling-spirals, such as are clinically 
used for cooling parts of the human body 1 ). 
Thb external shape of the coils appears from fig. 1; the polar 
flanges are conically truncated in order to make the interpolar space 
more accessible. When, however, the highest degree of magnetisation 
is required, two extra detached polar coils may be fixed around t e 
pole-shoes; in accordance - with a well-known rule of Kikchhqf* s 
the purpose of this is to saturate the polar pieces as far as possi 
this principle was, moreover, already applied by H. Lehmann in 
investigation (loc. cit. p. 424). We then obtain 100 (120) kiloampere- 
l ) Cooling the wire-coils themselves, e.g. by circulating oil is certainly desirabl > 
but rather elaborate and expensive. 
