Determination oftlw Earth's Horizontal Magnetic Force 401 



slight apparent increase of moment at one or more of the stages. In 

 fact, in an old magnet not recently remagnetised, and in a new magnet 

 which had been put through a previous temperature experiment only 

 three days before, the moment appeared greater on the third occurrence 

 of the " hot " temperature than on the first. 



In investigating these permanent changes of magnetic moment I 

 received valuable assistance from Mr. R. S. Whipple, then assistant at 

 Kew Observatory. 



§ 37. There are several other sources of uncertainty affecting tem- 

 perature coefficients. For instance, a small fall of temperature may 

 not always have an effect exactly equal and opposite to that of an 

 equal rise. 



Again, it is conceivable that the values of q and q may depend to 

 some extent on the age or strength of the magnet, and so may alter in 

 course of time. On this point I have some interesting data. 



In 1894: three collimator magnets after long absence in India were 

 sent to Kew Observatory to be reported on. The opportunity was 

 taken to redetermine their temperature coefficients — originally deter- 

 mined in 1865 — before sending them for repair to the maker. After 

 their return they were remagnetised and tested as usual. 



Table XIII gives the values deduced for 



1 dm _ ,, 



m tit 



at 0°, 15°, and 30° C, employing the values found for q and q'. 



Table XIII. 



Date. 

 Temperature. 



1865. 



r 



0° 



1894. 







1895. 

 \ 



15° 



3(? 



0° 



15° 



30° 



15° 



30° 





240 



264 



288 



252 



285 



318 



347 



363 



380 





219 



234 



250 



158 



200 



243 











290 



359 



429 



327 



366 



405 



363 



407 



451 



Mean for all 



250 



286 



322 



246 



284 



322 









„ „ i and iii. 



265 



312 



359 



289 



326 



362 



355 



385 



415 



Table XIY gives particulars as to the values of the moments of these 

 magnets at the times of the temperature experiments, and the loss of 

 moment which occurred between 1865 and 1894. 



The numbering of the magnets is arbitrary, but is the same in both 

 tables. Magnet ii was discarded in 1894, being slightly chipped. It 

 will be seen that the agreement between the mean temperature effects 

 in 1865 and 1894 could hardly be better, notwithstanding the large 

 diminution in the magnetic moments. 



