378 



Dr. C. Chree. Collimator Magnets and the 



ends of the magnet, and of a brass stirrup arrangement which carries 

 the magnet and affords the means of supporting, parallel to it, an 

 auxiliary solid brass cylinder. This brass cylinder is a regular geo- 

 metrical object, whose moment of inertia can be calculated from its 

 weight, length, and diameter. The actual inertia experiment consists 

 in observing the times of vibration of the magnet, under the earth's 

 horizontal force, when the auxiliary bar is in the stirrup, and when it 

 is removed. To reduce the possible effects of variation in force or 

 temperature, four complete series of vibrations are made, the first and 

 fourth without, the second and third with the auxiliary cylinder. 

 Allowance is made for the departure of the mean temperature from 0° C. 

 It is customary to make two independent determinations of the moment 

 of inertia, usually on different days, and in the event of serious dis- 

 crepancy a third experiment is made. In all the older experiments 

 conversion from British to C.G.S. units was necessary. 



§ 5. Coefficient P. — The meaning of this has been already explained 

 generally. I need only add that in the deflection experiment the axes 

 of the two magnets are perpendicular, and the centre of the deflected 

 or, as it is called, " mirror " magnet lies on the axis produced of the 

 collimator magnet. The general expression for the couple exerted in 

 such a symmetrical position, the centres of the magnets being at distance 

 r, is 



2mmV- 3 (1 + Pr- 2 + Qr~ 4 + ), 



where P, Q, are constants, whose values depend on both the 



deflecting and deflected magnets. As r increases, the terms involving 

 the higher negative powers of r tend to vanish relative to the first 

 term ; and in the present case it is assumed that the term involving P 

 is the last that need be retained. When this is true we can determine 

 P by comparing the couples answering to any two different values of 

 r. The distances originally adopted at Kew Observatory when British 

 units were employed were 1 foot and 1*3 feet; the distances now in 

 use are 30 cm. and 40 cm. For several reasons, I have " recorded the 

 value not of P but of P/r 2 at 30 cm., allowing in cases where British 

 units had been used for the small difference between 30 cm. and 1 foot. 



§ 6. The number of makers of collimator magnets is not large , 

 and the differences between the patterns are mostly small. Still 

 it seemed undesirable to wholly disregard the differences that 

 unquestionably exist. Accordingly, in summarizing the results con- 

 tained in the Observatory records, I have divided the magnets into 

 six groups, distinguished by the letters A to F. Four of the groups, 

 A, B, C, and E, contain magnets from one maker only, and are presum- 

 abty fairly homogeneous. Group D is the most miscellaneous, contain- 

 ing magnets by three if not four makers ; group F contains magnets 

 from two makers only. 



