12 



ADVANCED ELECTRICITY AND MAGNETISM. 



silk fiber 



great interest in that it brings the abstract discussion of Arts. 

 4 to 9 into relation with actual experiment. 



Gauss's method involves two independent sets of observations 

 with two different arrangements of apparatus, as follows : 



First arrangement. A bar magnet is 

 suspended, as shown in Fig. 13, at the 

 place where the value of H r is to be de- 

 termined, and set vibrating through a 

 small amplitude about the suspending fiber 

 as an axis; and the time / of one com- 

 plete vibration is observed. Then from 

 equation (3) of Art. 1 1 we have : 



(i) 



The moment of inertia K of the bar magnet is known, being 

 determined from the measured dimensions and mass (in grams) 

 of the bar. Therefore the quantities m, I* and H r , only, are 

 unknown. 



N 



Fig. 13. 





<j_._. 



N 







dg magnet 



top view, 



Fig. 14. 



8 small magnet 



Second arrangement. A very small magnet, ns, Fig. 14, is 

 suspended by a fine silk fiber at the place occupied by the large 

 bar magnet in the first arrangement. This small magnet ns, 



* Of course J is somewhat less than the length of the bar. 



