THE EARTH’S MAGNETIC FIELD IN INTERNATIONAL UNITS. 
445 
axis of the magnet not being perpendicular to the axis of the coil, is proportional to 
the cosine of this angle, there will be no correction in the case of the longer magnet. 
The readings obtained with the shorter magnet will, however, require to be corrected 
on this account, that is, the values of H calculated from the deflections made with 
this magnet will require to he multiplied by cos 38'. This correction amounts to a 
little more than one unit in the fifth place in H. 
Adjustment 3 was made by supporting a scribing block so that tlie pointer was on 
a level with the centre of the coils and then adjusting the jiosition of the coils, with 
reference to the magnetometer, till the edges of the flanges just cleared the pointer 
when the coils were rotated. This adjustment was further checked by means of a 
right-angled slider, which rested on the deflection bar. This slider was brought to 
bear against a flat-headed screw, H fig. 1. The heads of these screws were adjusted 
by means of distance pieces and a straight edge to be at the same distance from 
the mean planes of the two coils. When the adjustment was complete, it was found 
that the reading of the slider on the deflection bar was the same of the two sides. 
It was found, by rotating the magnetometer when a plummet was suspended in 
jilace of the magnet and viewing the fibre just above the plummet with a telescope, 
that the axis of the fibre did not differ from the axis about which the coils turned 
by more than half a millimetre. The height of the magnet was adjusted by bringing 
its centre to lie on the axis of the sighting tube which was used in adjusting the 
axis of the coils. 
Since making one of the above adjustments generally affected the others, a method 
of successive approximations had to be adopted. This was continued until the 
centre of the magnet did not differ from the centre of the coils by a millimetre, 
while the axis of the coil was accurately horizontal, and the axis about which it 
turned vertical. 
Fig. 4. 
General Arrangement of Circuits .—The general arrangement of the circuits 
employed in the experiments is shown diagrammatically in fig. 4. In this figure B 
represents a battery of 14 accumulators, and D is a switch, by means of which 
