460 Prof. W. A. Jenkins on 



it is necessary to make an allowance for this in the calculations. 

 The angles between the mirror faces can be accurately 

 measured and, from the values found, the angles of deflexion 

 1 and 6 2 in the two parts of the experiment calculated. 

 Then the following equations will give the value of H : — 



Fcos6' 1 = Hsin^ 1 , 

 Fcos0 2 =(X-H;sin0 2 , 



where X is the field of the solenoid. 



H _ x 



tan #! cot #2 + 1- 

 In the experiment actually carried out, 

 0. 1 =44° 2iV -60" 



e 2 =m° ±2' 15" 



yi = 13-362 

 cos «=-99986 



C = -037547 ampere 

 X = -62955 

 H^-3245 



The experiment was carried out ten days after the former 

 one, and hence coincidence of results could not be expected. 



Errors. — Possibilities of errors not previously described 

 arise in this experiment owing to the following causes : — 



(1) A lateral displacement of the subsidiary magnet from 



its true position opposite the centre of the suspended 

 one. 



(2) An angular displacement of the subsidiary magnet 



from its true position perpendicular to the solenoid. 



(3) Alteration of magnetic moment of the subsidiary 



magnet. 



(1) The following calculation shows the error likely to 

 arise from a lateral displacement of the subsidiary magnet. 



Suppose the magnet is displaced 1 cm. from its correct 

 position. 



Then a simple calculation shows that an error of 1 part 

 in 3000 is introduced if, as was the case in the actual experi- 

 ment, d is about 50 cm. and I about 18 cm. If the displace- 

 ment is not more than 3 mm. the error is not more than 1 part 

 in 10,000. The subsidiary magnet can easily be placed to 

 within 3 mm. of its correct position. 



(2) An angular displacement of 1\° would introduce an 

 error of 1 in 10,000. The magnet can be placed in its 

 correct position to a greater degree of accuracy than this. 



(3) The only factor likely to cause a sudden change of 



