1892.] testing the magnetic properties of iron and steel. 333 



Near its top the wire carries a small magnet G whose axis is at 

 right angles to the wire. The lower end B of the wire carries a 

 small fork BBE, also of aluminium wire, across which the silk 

 fibre BE is stretched. Attached to this fibre by means of wax is 

 a small plane mirror F, such as is used in reflecting galvanometers, 

 carrying a small magnet whose axis is at right angles to the fibre 

 BE. Attached to the bottom edge of the mirror is a disk of 

 thin mica about 1 inch in diameter. When the plane of the 

 mirror is vertical, the plane of the disk is horizontal. Close 

 beneath the mica disk is placed a piece of cardboard in a horizontal 

 position. The mica disk, owing to the close proximity of the 

 cardboard, very rapidly reduces the mirror to rest. The mirror is 

 fixed to the fibre so that the centre of gravity of the mirror and 

 mica disk is slightly below the line BE. Thus the controlling 

 force acting on the lower system consists partly of gravity and 

 partly of the magnetic force due to the earth and to any control 

 magnets which may be required to bring the mirror into any 

 desired position. The mirror now possesses two independent mo- 

 tions, the one about the axis AK and the other about the axis BE. 

 The apparatus is set up as in fig. 3*, in which the suspended 

 part has been turned through a right angle so that the mirror is 



K 



n c 



Fig. 3. 



I 



now seen edgeways. The plane of the paper is supposed to be a 

 plane through the wire AB at right angles to the magnetic 

 meridian. The magnet G is therefore at right angles to the plane 

 of the paper. The mirror F is shown slightly tilted. The coil L 



* This figure is purely diagraniatical and does not represent the relative pro- 

 portions of the separate parts of the apparatus. 



26—2 



