704 



in the cylinder cannot have any influence in our experiment, their 

 sole effect being a slight retardation of the magnetic reversals. So 

 far as we can see, the above effects are the only ones that have 

 the same frequency as the current in the coil and are therefore 

 magnified by resonance. When now the coil was connected to the 

 main alternating current conductors the image on the scale remained 

 perfectly at rest so long as the length of the suspending wire was 

 not such as to make the frequency of a free vibration of the cylin- 

 der coincide very nearly with that of the alternating field. The 

 resonance appeai'ed and disappeared again by a change of length 

 of the wire by 1 mm, the whole length being 8 cm. 



Ill order to find tlie length required for resonance and to make 

 sure that the suspended apparatus did not vibrate in one of its 

 higher modes, we used the following method by which we could 

 also determine the moment of inertia of the cylinder. 



At the lower end of the iron cylinder we sealed a short copper 

 cross bar whose moment of inertia was 10,7. 



For the moment of inertia of the cylinder calculation had given 

 0,0045. 



It follows from this that the period of oscillation of the cylinder 



becomes / '■ — = 48,8 times greater bv adding the small cross 



K 0,0045 & . Ö 



bar. If therefore we chose the length of the wire so as to have a 

 frequency J ") with the cross-bar, the frequencj' without it would 

 be about 48,8. This is nearly equal to the frequency of the alter- 

 nating current. 



We were sure by this that the suspended system would vibrate 

 in its fundamental mode. In order to determine the moment of 

 inertia more accurately however, the cylinder was now placed 

 within the coil and the length of the wire was increased until the 

 resonance was at its maximum. Then the frequency of the free 

 vibrations might be supposed to be equal to that of the alternating 

 current which was found to be 46,2. After this the arrangement 

 was removed from the coil and the cross bar fixed to it. We then 

 found the frequency 1,14. From these numbers we deduce 



= 10,7 M — =0,0065. 

 1.46,3; 



After these preparations it was found that Effect II, i. e, the 

 oscillation caused by [)ei'manent poles in the cylinder, was of no 



') By frequency we always mean the number of complete oscillations iu a 

 second.' 



