402 Dr. C. Chree. Collimator Magnets and the 



Table XIY. 



Magnet 



Moment in 

 xooo. 



Moment in 



Per cent, loss 

 in 29 years. 



Moment in 

 lovo. 



i 



936 



568 



39 



863 



ii 



776 



464 



40 





iii 



945 



718 



24 



855 



On the other hand, the values found for q and q' in 1895 — values 

 based in each case on two consistent experiments — are unmistakably 

 higher than those found in 1865 and 1894. There is no apparent 

 reason for this except the fact that the magnets had been remagnetised. 

 The makers, in reply to inquiries, stated explicitly that " the magnets 



. . were only cleaned." 



We seem driven to the conclusion that whilst a gradual loss of mag- 

 netic moment may not appreciably influence the values of the tempera- 

 ture coefficients, the remagnetisation of a magnet does influence these 

 coefficients, in at least some cases. 



§ 38. Induction Coefficient. — The method of determining /x has been, 

 already described in § 3. There are here also several uncertainties. 



The assumption that the relation between the temporary moment 

 and the field is strictly linear may not be sufficiently exact; 0*44 C.G.S. 

 unit is a considerably stronger field than Lord Eayleigh found to limit 

 the linear part of his induction curves, and recent German observers 

 seem disposed to narrow his limits. The accuracy of the assumption is 

 doubtless less in some collimators than in others, but I have no data on 

 which to proceed. 



* Again, it is open to doubt whether a magnet possessed of a large 

 permanent moment responds equally to the action of two small fields, 

 one tending to increase, the other tending to diminish, the total 

 moment. This uncertainty enters into the application of ft as well as 

 into its original determination. For in the vibration experiment the 

 temporary and permanent magnetisms are in the same sense, whereas 

 in the deflection experiment they are in opposite senses. 



If the presence of a permanent magnetic moment influences the value 

 of /x, then the value found at Kew, even if otherwise above criticism, 

 will presumably become less exact as the moment falls off. 



Another source of uncertainty is the probability that fx varies with 

 the temperature. 



We can best judge of the possible influence of these sources of uncer- 

 tainty by reference to the formula 



* Lament, c Hand ouch des Erdmagnetismus,' pp. 149 — 150, says /j. is less for 

 strengthening than for weakening fields, especially when the permanent moment is 

 very large. 



