( 663 ) 
paratively long core by heat could distinctly alter the comparatively 
short distance between the poles. 
c. Cryogenic apparatus. As it was necessary to shut off from the 
air the space in which the ellipsoid was freely suspended since it 
contained liquid hydrogen and its vapour, a fairly complicated cryo¬ 
genic apparatus had to be employed. This is shown diagrammatically 
in PI. II fig. 1 and in section in fig. 3. The apparatus consists 
chiefly of three tube-shaped portions which, naming from outside 
inwards, we call the cover, the adjusting tube /, and the holder b. 
The cover consists of a silvered vacuum tube A, a brass tube B, 
a glass tube C, and a cap D which shuts off the apparatus from 
the air. 
Holder. The ellipsoid a (figs. 3 and 5) can turn round a vertical 
axis with the holder b in which it is fixed. For the greater part 
of its length the holder is made from a tube 6, of german 
silver — a substance that is rigid, little magnetic, and a bad heat- 
conductor. The lower end is joined to a copper rod b x , which has 
only a very weak inherent magnetism. The holder is connected to 
the rod k by the spiral spring g x [g x was used for iron and cobalt; 
the weaker spring g % , which was used for nickel and magnetite is 
shown at the side). To make the equilibrium stable ar.d to prevent 
the ellipsoid from being attracted to the poles of the magnet the 
holder is held fast underneath by a wire of platinum-iridium of 
0.1mm. diameter, for the torsion of which a correction need hardly 
be applied ($ 4). 
The tube b t and the rod b x are carefully adjusted on the lathe, 
and the ellipsoid a (fig. 4) is fixed carefully in a cylindrical opening, 
the' diameter of which is equal to the minor axis of the ellipsoid. 
If the ellipsoid is nickel or magnetite it can be fixed in position 
with a little wax. With iron and cobalt, however, the ellipsoid is 
subject to such strong forces that it is necessary to clamp it fast by 
covering it with a thin piece of sheet copper and then driving it 
forcibly into the opening. The turning of the ellipsoid is transmitted 
through the rod b x , and the thin-walled german-silver tube 1 ) b 2 to 
the mirror k. From the mirror through the opening /„ and the 
window C., (figs. 1 and 3) the torsion of the spring g x is read. A 
i) A slight twisting of this tube is of no account. Only that portion of the 
apparatus between the mirror and the cap acts as a spring. Twisting of the 
portion of the apparatus * below the mirror only transmits the couple to that 
spring, its sole effect is to slightly, but not noticeably, alter the azimuth of the 
magnet. 
