of Robison ball-ended Magnets. 
187 
which is used to deflect the magnetometer, is placed on the 
drawing board, each of its spheres resting in a conical depression 
formed in a heavy brass disk. These disks prevent the magnet 
from rolling and also raise it to the same level as the suspended 
magnet. The positions of the centres of the spheres are recorded 
on the board by pricking the board by a pin passing through a 
small hole at the bottom of the conical depression. 
The magnetometer is first put into position on the board and 
when the suspended magnet has come to rest in the magnetic 
meridian, the positions of its poles are observed and recorded, the 
deflecting magnet being removed to a distance. 
The deflecting magnet is then placed near the magnetometer. 
Any position may be chosen as the theory of the experiment is 
quite general, but the best position is that in which the axis of the 
deflecting magnet passes approximately through the axis of 
suspension of the deflected magnet, the axes of the magnets being 
roughly at right angles. The deflexion should not be less than 
30°. The positions of the poles of both magnets are then recorded 
on the board. 
Let.d. 0 , B 0 (Fig. 4)be the projections of the poles of the suspended 
magnet, when undeflected, A 0 being the north pole. Then A^B^ 
marks the magnetic meridian. Let A, B be the projections of the 
poles of the magnet when deflected and let C, D be the projections 
of the poles of the deflecting magnet, C being the north pole. Let 
0 be the projection of the centre about which AB turns ; this 
point is determined by the intersection of A^Bq and AB. 
13—2 
