Be 
Vertical force Magnetograph. agd 
increases with rise of temperature, the couple due to the 
torsion of the fibre will increase. On this account also the 
north end of the magnet will rise. Next let us adjust the 
balance of the magnet so that the centre of deat lies on 
the same side of the axis of the fibre as the south pole, the 
displacement being such that to make the magnet lie with its 
axis horizontal, the fibre has to be twisted in the anti-clock- 
wise direction. In this case, when the temperature rises 
the north end of the magnet will tend to rise owing to the 
decrease in magnetic moment, but will tend to fall owing to 
the increase in the stiffness ue the fibre. Thus, by suitably 
adjusting the horizontal displacement of the centre of gravity 
o 
of the magnet, that is the initial torsion of the fibre, we can 
so arrange matters that the decrease in the couple due to the 
one effect is exactly equal to the increase due to the other; 
and so changes of temperature do not affect the position of 
the magnet. In the instrument described, the horizontal 
displacement of the centre of gravity is effected by moving a 
small weight along the magnet; and the possibility “of 
obtaining complete compensation depends on the fact that 
the ppeticicut of increase of the rigidity of the fibre is much 
greater than the coefficient of neue expansion of steel™. 
=e Spat of the instrument will be evident from 
figs. 1,2,3,4(Pl. XXII.). The base of the instrument consists 
of a Soe metal casting having uprights at the ends which 
earry the attachments for the ends of the fibre. ‘Two uprights 
screwed to the middle of the base serve to support a circular 
plate U which carries the plano-convex lens L, used to form 
an image of the slit on the recording-drum. The circular disk 
V can turn through a small angle, its position being deter- 
mined by two adjusting screws, andit carries the right-angled 
reflecting prism P. One end of the quartz fibre is fused to a 
quartz spring G, while the other end is fused to a small rod 
of quartz which is soldered into a small metal rod, and this 
rod is clamped by means of a screw to the screw H. Both 
the screws H andI have a key-w ay cut along them, so that 
by means of the nuts O they can be moved ‘parallel to their 
axis but without rotation. These screws pass through holes 
in the two collars m and M, which themselves fit in the 
uprights carried by the base. The collar M carries a divided 
head, and is fitted with a fine adjustment Z, shown in fig. 3. 
Since only the portion FB of the quartz thread is twisted, 
the fine adjustment and divided head allow of the twist put 
* The simple rigidity of fused silica in the form of fibres of moderate 
diameter increases by ‘00013 of its value for 1° C., while the coefficient 
of linear expansion of steel is ‘000011. 
