376 Dr. C. Chree. Collimator Magnets and the 



Sect. 



32. Uncertainties in determination of moment of inertia. 

 33 — 34. torsion. 

 35 — 37. „ ,, temperature coefficients. 



38. „ ,, induction coefficient. 



39 — 44. Asymmetry in magnets. 



45 — 47. Law of action between magnets. 



4S — 50. Concluding remarks. 



§ 1. The present paper deals with magnets employed in measuring 

 declination and horizontal force. More than 100 collimator magnets 

 of English make have been examined at Kew Observatory, and the 

 record of the results forms probably a unique mine of information. So 

 far as I know, the only use hitherto made of this has been in the com- 

 pilation of a statistical paper on the mean and extreme values of the 

 temperature and induction coefficients by the late Mr. G. M. Whipple.* 



The examination of a collimator magnet at Kew Observatory con- 

 sists mainly in the determination of certain " constants." These are 

 the "temperature coefficients" q and q', the "induction coefficient" /x, 

 and the " moment of inertia " K. 



The values found for these " constants " are utilised in the construc- 

 tion of tables, intended for reducing the observations of horizontal force. 

 After the tables are constructed one or two observations are made. 

 Their primary object is to ensure that the application of the tables 

 leads to satisfactory results, and that there are no instrumental defects ; 

 but incidentally they afford the means of determining the magnetic 

 moment, m, of the collimator magnet, and also the value of a " constant," 

 P, appearing in the expression 



2m7?A-- 3 (l+P?'- 2 ) 



for the couple exerted by the collimator magnet on an auxiliary magnet, 

 moment m", at distance r. The investigations described in the present 

 paper have been prosecuted at intervals during the last five years, as 

 the pressure of other work allowed. Their object has been twofold, 1° 

 to find out whether any relationships exist between the several con- 

 stants, and 2° to ascertain where our present knowledge wants 

 amplification, and where the present tests are least satisfactory. 



I shall first explain the real significance of the " constants," and 

 describe briefly the method of determining them. 



§ 2. Temperature Coefficients. — It is assumed that the magnetic moment, 

 m , at a temperature of t° C, the magnet being free from external force, 

 is connected with the moment m at 0° C. by the relation 



m'/ra = 1-qt-q'P (1), 



where q and g/ are absolute constants for the magnet concerned. If (I) 

 * 'Boy. Soc. Proc.,' vol. 26, pp. 218-222, 1877. 



