the Determination of "A." 



459 



the accuracy with which the angle of deflexion can be 

 measured. A similar measurement is involved in the Kew 

 Magnetometer method, and the accuracy of the two methods 

 may therefore be said to be the same. 



Second Method. — The principle of this method is very 

 similar to that described in the previous paper. An artificial 

 Held is produced in the solenoid opposite in direction to, and 

 in intensity twice that of the Earth's field. The equality of 

 the two fields is tested by means of the deflexions caused by 

 a subsidiary magnet piaced in the Tangent B position of 

 Gauss. 



Details of the Experiment. — The tube was first set accu- 

 rately parallel to the Earth's field. The octagonal shaped 

 framework suspended underneath the magnet had mirrors 

 attached to all its faces and the adjustments were made so 

 that the scale was reflected from face D (diagram 2 a). 



The external magnet sliding in a groove at right angles to 



Diagram 2. 



JC 



/ \ 



y 



{ 





1 







D 



a. Position under influence of Earth's field alone. 



b. Position under influence of Earth's field + that of external magnet. 



c. Position under influence of Earth's field + external magnet 4- solenoid 



field. 



the solenoid was then adjusted until the scale was reflected 

 from face C, diagram 2b, into the telescope. Without 

 moving the external magnet the solenoid field was switched 

 on aucl adjusted in strength until the position of the 

 suspended magnet was as in diagram 2 c. Then the image 

 of the scale was reflected from face B into the telescope. 



If the octagonal framework is accurately made and the 

 angles between the adjacent faces are 135° exactly, then 

 obviously the solenoid field is exactly twice the Earth's field. 

 and we oet 



2H = 47niO,cos«. 



The measurement of H is now simply a measurement o\^ 0. 

 In practice, however, the angles are not exact!)' 135°, and 



