Determination of the Earth's Horizontal Magnetic Force. 38 X 



made on the special magnets the appendages contribute only from 25 

 to 30 per cent, of the value of K. 



Again, the size of the appendages is determined mainly by that of 

 the auxiliary brass bar, which according to the Observatory records is 

 nearly constant for magnets of the same make. It is thus improbable 

 that variations in the appendages exercise a large influence on the 

 values of 7r 2 K in the several groups. 



§ 8. Taking all the data into consideration, I conclude from Table I, 

 that the magnets of group A are on the whole distinctly the largest, 

 and those of group F very distinctly the smallest ; that the other four 

 groups are very fairly similar in this respect, though the average 

 magnet of group E is almost certainly a trifle smaller than that of 

 groups B and C. • The variability in size was really one of my princi- 

 pal reasons for grouping the magnets. The values of m and fi should 

 increase, ceteris paribus, with the size of the magnets, and an analysis 

 which overlooked the difference between groups A and F might well 

 prove misleading. So far as individual groups are concerned, I do 

 not think that the neglect of possible variations in the size of the 

 magnets is likely to prove serious. My reasons for this opinion are 

 partly based on phenomena discussed later in §§ 15 and 16. 



§ 9. Before proceeding further, a second source of uncertainty must 

 be noticed. The moment of the residual magnetism in a magnet is 

 not determined solely by the magnetic quality of the steel, it depends 

 on the conditions under which magnetisation took place, on the time 

 elapsed since that event, and on the usage to which the magnet has 

 been exposed. 



In the present case I have strong reasons to think that the very large 

 majority at least of the magnets were magnetised under a nearly uni- 

 form set of conditions. Until quite lately it was the invariable 

 practice to magnetise all the magnets at the Observatory itself, and the 

 same coil with similar battery power has been in use for the last forty 

 years. The magnet is not placed inside the coil, but is stroked in a 

 uniform way on the end of a very massive slightly projecting iron core. 

 The capacity of this core to hold out a heavy pole piece has been re- 

 garded as a criterion of the battery being in proper order ; and as 

 apparent " saturation " is reached in an ordinary collimator magnet 

 with the battery below standard condition, the test, though a rough 

 one, appears fairly satisfactory. 



There is a variety of indirect evidence as to the uniformity of the 

 conditions. For instance, the dates of magnetisation of the ten magnets 

 of largest m in group C, arranged in descending order of magnitude, 

 were as follows :— 1884, '66, '79, '73, '95, '84, '83, '94, '75, '89, '68. 



As regards possible loss of magnetic moment, the magnets, with one 

 or two probable exceptions, had all been magnetised but a short time 

 before m and the various " constants " were determined. 



