the Determination of"H" 4z57 



the centre of the solenoid by the diameter of the end of the 

 helix. Substituting the values n = 13*362, cos a. = '99986, 

 we get H = -3236. 



Accuracy obtainable. — The following sources of error were 

 discussed in the previous paper : — 



1. The magnetic axis of the solenoid not being horizontal. 



2. The magnetic axis of the solenoid not being parallel to 



the direction of '* R." 



3. Irregularity in the winding. 



4. The longitudinal displacement of the magnet from the 



centre of the tube. 



5. Residual torsion of the fibre. 



6. Inaccurate determination of the period of oscillation. 



7. Inaccurate determination of the current. 

 S. An error in the determination of n. 



9. Heating of the coil by passage of the current. 



It was there shown that with the exception of 6, 7 and 9, 

 the sources of error were negligible. 



In the present experiment 6 is not present at all, while as 

 the current passing is only one half of what it was in the 

 previous method, the heating effect is diminished to \ of 

 its previous value. As by taking proper precautions it was 

 previously negligible, it is more so now. The determination 

 of the magnitude of the current still remains one of the chief 

 factors limiting the accuracy obtainable, but with standard 

 resistances and a good galvanometer an accuracy of 1 part 

 in 10,000 is not difficult to obtain. Error 5, due to the 

 torsion of the fibre, is negligible when an extremely fine fibre 

 is used. In the present experiment, however, when prolonged 

 oscillation of the magnet is not only not required, but is 

 undesirable, a thicker suspension fibre possesses considerable 

 advantages. Consequently a moderately thick suspension 

 fibre was used. A stronger, heavier magnet system was 

 also inserted in the holder, and on putting into the fibre 

 1440° of torsion a deflexion of sin -1 *012 was observed. 

 The following equations give the maximum allowable value 

 of the angle of deflexion due to the residual torsion for an 

 accuracy of 1 in 10,000. 



Suppose 6 of torsion remain in the fibre and n be the 

 coefficient of the torsion. 



2EU sin a = n (6 — a), where a. is the deflexion due to 

 torsion. When the tube is rotated through 90° and the 



