TERRESTRIAL MAGNETISM 303 



on the circle. Now turn the needle over about NS as axis and 

 Mi-pend it with the other face up, and let (6), Fig. 223, be the plan 

 in the new position. NS is still in the same direction, but AB is 

 thrown as much on one side of NS as it was previously on the 

 other, and the mean of the two readings for the direction of AB 

 gives the direction of NS. 



The form of the apparatus adopted in this country is known as 

 the Kew magnetometer. The needle is a hollow magnetised steel 

 tube about 4 in. long and J in. diameter, Fig. 224, with a very fine 



FIG. 224. 



horizontal scale on glass at one end y, and a lens I with s as its 

 focus at the other. It is provided with two points of suspension, 

 p l and p.,. The needle is hung up in a torsion box by a silk fibre, 

 as represented diagram matically in the figure. This torsion box is 

 mounted so that it can be turned about a vertical axis, easily 

 adjusted vertically by levelling screws and a level not shown. A 

 telescope focused for a long distance away moves round the same 

 ;i\i^. and i> provided with verniers on arms moving on a horizontal 

 divided circle c. First, it is necessary to eliminate the torsion of the 

 fibre. This is done by placing the torsion box as nearly as possible 

 in the magnetic meridian, and then hanging a non-magnetic plumb 

 bob of the same weight as the magnet and in its place. When the 

 bob has come to rest the magnet is hung up by p v and the telescope 

 i*. moved round so that the centre of the scale *, viewed through 

 the window zc^ and lighted through the window w 2 , is seen on the 

 rrovs wire of the telescope. The axis of figure is the line joining 

 the centre of the scale s to the centre of the lens /. The readings 

 O f the telescope verniers are then taken. Say that the mean reading 



