288 



Mr. E. P. Adams on the Electromagnetic 



edge and near the upper edge of the mica, so as to form an 

 astatic system. The needles are placed perpendicular to the 

 axle. The mirror is also cemented to the mica a little above 

 its centre, and is observed through an opening in the brass 

 tube at S, covered with a thin glass plate. The whole needle- 

 system is suspended by a quartz fibre 32 cms. in length. 



Fig. 2. 



The magnetometer-tube H is carried in a brass collar P 

 which is screwed to the brass plate M. This plate is pro- 

 vided with levelling-screws, and rests on the board-shelf N, 

 supported at its ends on two brick piers. Underneath the 

 levelling-screws are placed pieces of felt, to take up any 

 mechanical vibrations. By means of control-magnets placed 

 on the brass plate M, any required degree of sensitiveness can 

 be given to the needle-system. All metallic parts of the 

 magnetometer-tube and supports are earthed. A single turn 

 of wire at K serves to determine the needle-constant. The 

 deflexions are read by means of the telescope and scale T, 

 placed at a distance of 3 metres from the mirror. 



