Physics. — 11 An improved semicircular electromagnet 1 *. By Prof. 
H. E. J. Gr. du Bois. (Communication from the Bosscha- 
Laboratory). 
(Communicated in the meeting of June 26, 1909). 
Starting from my theory of the split-ring magnet and its experi¬ 
mental confirmation by H. Lehmann 1 ) I designed a large ring- 
electromagnet in 1894, which I believe may claim to be considered 
the prototype of various apparatus since described *). Since the general 
introduction of magnetically efficient cast material it proved feasible 
to deviate from that simple shape and to redesign it (in 1898) as a 
so-called semicircular magnet 3 ); this was much lighter and handier 
in many respects and a great number of this type have since been 
in use in laboratories. 
Within a cylindrical “interferric” space of 1 mm. length and 
6 mm. diameter their field amounts to 38—40, averaging rather 
more than 39 kilogauss, in consequence of the somewhat variable 
properties of cast material. After some ten years of varied experience 
it now again appears desirable — as indeed it does for most kinds 
of instruments — to further revise the design with a view to practical 
use. As the resources of a well-equipped experimenter will in many 
cases enable him to do good work within narrower interferric spaces, 
1 could attempt to increase the field up to 50 kilogauss without 
exceeding a total weight of about 300 kg. — distributed over three 
equally heavy pieces; the apparatus remaining sufficiently easy to 
handle and carry about in laboratories. 
In the meantime Prof. Pierre Weiss described an electromagnet 
of great power 4 ), which uses up to 18 kilowatt and weighs about 
1300 kg.; by this, however, probably only a few privileged labora¬ 
tories are benefited, and it appears, moreover, less adapted for the 
important class of modern magneto-optic investigations on account of 
the long path of the rays of about 125 cm. I could, however, partly, 
avail myself of his theoretical considerations and results as well as 
of some details of his design. 
Fig. 1 is reproduced at a tenth of the real size. The base part 
TH. du Bois, Wied. Ann. 46, p. 485, 1892. - H. Lehmann, Wied. Ann. 48, 
p. 406, 1893. 
2 ) H. du Bois, Wied. Ann. 51, p. 537, 1894. 
3 ) H. du Bois, Verh. Berl. physik. Ges. 17 , p. 99, 1898; Zeitschr. fur Instr.kunde 
19, p. 357, 1899; Ann. der Physik. (4) 1, p. 199, 1900. 
*) P. Weiss, l’Eclairage electr. 15, p. 481, 1898; Journ. de Phys. (4) 6, p. 353, 
1907. — G. Zindel, Electrotechn. Zeitschr. 30, p. 446, 1909. 
