( 662 ) 
The law according to which this magnitude changes shows that 
the change is not a consequence of the non-uniformity of the field; 
for just when the disturbance reaches its greatest value, the field is 
most regular owing to the closer approach of the flat pole-pieces. For 
distances of 23 mm. and greater the influence is insignificant, and 
the couple is constant. 
b. Electromagnet. From what has been said about the influence 
of the ellipsoid and the pole-surfaces it follows that the distance 
between the pbles should be about three times the length of the 
ellipsoid. The total thickness of the four walls of the Dewar tubes 
and of the holder $ 2c) could not be made smaller than 5 mm. 
Hence, keeping account of the difficulty on the one hand of obtaining 
strong fields of wide extension and on the other hand of reducing 
very small ellipsoids to the correct form, we decided upon an inter¬ 
pole distance of 9 mm. and a length of 3 mm. for the ellipsoids. 
With this distance comparatively strong fields (up to 25000 gauss) 
may be excited with a magnet whose cores are 9 cm. in diameter. 
The electromagnet of this power which was used in these experiments 
has already served for magnetic experiments at high temperatures. 
It has already been described*) and is represented diagrammatically 
in fig. 1 PI. II. Comparatively light (132 KG.) and, taking its power 
into account, easily transported, it was possible to study it in Zurich 
and to use it in Leyden. Hand-wheels, whose position is read from 
divided circles, communicate a horizontal micrometric movement to 
the pole-pieces. 
The magnet turns upon a vertical axis and for that purpose is 
mounted upon a ball-bearing support. The azimuth is determined by 
means of a fixed mark on a cylindrical scale E 2 attached to the 
movable portion of the supporting base. Each of the coils has 1500 
turns of 2.5 mm. wire and has a resistance of about 2 ohms. As 
the coils are arranged for a current of 10 amp. under ordinary 
circumstances, and as the current can for a short time be increased 
to 25 amp. the number of ampere-turns at one’s disposal may reach 
as high as 75.000. The water circulation E & between the double 
wails of the coils has this immediate advantage that the duration of 
an experiment may be doubled, but it is chiefly of importance in 
protecting the pole-pieces from heat. Such a beating would lead to 
various difficulties, of which one of the worst would be that the strength 
of the field w ould noticeably change, for the expansion of the com- 
i) G. Zundel. Revue electrique 20 Join 1909 and Elektiot. Zeitschr. XXX, 
p. 446, 1909. 
