the Determination of"H." 455 



Earth's field, so that if is the angle of deflexion, 



2H.I sin = 2 F 7 cos 0, 



E = Fcot0/ 



A determination of F and 6 therefore gives the value of "H." 

 As before F is equal to -lirnO cos a, where n and « are 

 constants of the Solenoid and the measurement of F reduces 

 itself to the measurement of the current C. 



Diagram 1. 





I 

 i E 



1 



*s 



Diagram 1 illustrates the method adopted for measuring 

 the angle of deflexion. 



X Y is the coil accurately placed at right angles to the 

 Earth's field. 



A B is the un deflected magnet. 



An image of the scale E is reflected from the mirror 

 surface C into the telescope G. Suppose the zero of the 

 scale to be at the intersection of the telescope cross-wires. The 

 current is now switched on and the magnet deflected as 

 shown in the lower part of the diagram. The strength of 

 the current is then adjusted until the zero of the scale is 

 reflected from surface D to the intersection of the cross-wires 

 of the telescope. The angle of deflexion is then the angle 

 between the mirrors C and D. This is a constant quantity 

 and can be accurately determined. and D are silvered 

 microscopic cover-glass mirrors attached to the sides of an 

 octagonal aluminium framework which is hollowed out for 

 the sake of lightness. Similar mirrors are attached to all 

 sides of the octagon in order to preserve a symmetrical 

 distribution of the weight. The framework is attached to 

 the underside of the magnet-holder. 



2 H 2 



