Component of the Eartlis Magnetism at Oxford. 441 



resultant of translation vanishes. But the bracket in the ex- 

 pression for the couple forms a correction factor which must 

 be taken count of. The whole formula is then, 



f = , to n4( 1+ ^)( 1+ ^ sH < )r , 



The two magnets evidently act inductively on each other. 



The field about each due to the other is — ^ — . In speaking 



of the earth's induction I shall show how I evaluate this. 

 For my magnets the effect begins to be sensible at about 

 60 centim. 



Some details about the magnets. I have used Joule's 

 method for eliminating the earth's induction during vibration, 

 by fixing two magnets on one stem at such distance that each 

 neutralizes the earth's action on the other. The magnets 

 must be equal. They are similarly shaped out of the same 

 bar, and magnetized on the Gramme machine. This gives 

 them at once a larger charge than they can permanently 

 retain. 



If they are very unequal, one will be found to be not quite 

 hard. It is only by making them quite hard that they can 

 be got to take fairly equal charges. It is not much use 

 trying to make the charges perfectly equal, as the moments 

 continue to fall for a long time after magnetizing, and similar 

 treatment is more likely to make them equal in the long run 

 than attempts at subsequent magnetizing or demagnetizing. 



In the vibration experiments the two magnets, which have 

 small holes through their centres, are suspended on a brass 

 stem at the proper distance apart. 



The magnets weighed over 400 grammes each ; they were 

 suspended by about 6 feet of bifilar wire. I distrust silk in 

 vibration experiments. It never seems to give definite values. 

 The torsion of the wire used was determined by means of two 

 rectangles of lead, one having the weight of the double 

 magnet, and one that of the single one. The torsion was 

 42'0 C.G.S. units of couple for the larger, and 35*8 for the 

 smaller weight. There is no doubt about this difference of 

 torsions for different weights. It has been observed re- 

 peatedly. 



The moments of inertia of the magnets were determined by 

 comparison with a rectangular brass bar 30 centim. long, 

 having nearly the same weight as one of the magnets. The 

 moment of inertia thus obtained differed considerably from 



* See Postscript. 



