400 



Dr. C. Chree. Collimator Magnets and the 



the values of the magnetic moment, reduced to a common temperature, 

 at each of the three "hot," "mean," and "cold" readings. Thus, for 

 instance, if the three " hot " temperatures were 35°, 36°, and 37°, the 

 observed magnetic moments were reduced to the mean temperature 36'. 

 The departures from the mean temperatures in this case are so small 

 that trifling errors in q or q are immaterial. Corrections were care- 

 fully applied in all cases from the magnetograms for changes in the 

 horizontal force and declination. 



In all, sixteen experiments made on ten different magnets were 

 dealt with ; the mean results are given in Table XII. By ( - om/m) is 

 meant the diminution found in the magnetic moment divided b}^ the 

 original v alue of the moment at atmospheric temperature. This is multi- 

 plied by 10 6 to avoid decimals. 



Table XII. — Mean Values of ( - 8m jm) x 10°. 



Temperature. 2nd reading — 1st. 3rd reading — 2nd. 3rd reading — 1st. 



"Hot" 266 95 383 



"Mean" 260 75 346 



"Cold" 166 56 229 



Mean of 3 stages... 231 75 319 



In one case only one cycle was taken. 



One cannot measure m accurately to one part in 100,000, much less 

 to one part in a million, so that a high degree of accuracy can hardly 

 be claimed for the above figures. It seems, however, perfectly clear 

 that normally there is a loss of magnetic moment, but that at the same 

 time the average loss is very small. 



These conclusions were supported by a large number of other cases 

 which I examined, though less carefully. 



On the average, according to Table XII, a complete temperature 

 experiment causes a loss of about - 04 per cent, in the value of m, and 

 of this loss much the greater part occurs during the first temperature 

 cycle.* The true reversible temperature variation of m during a cycle 

 . averages very sensibly over 1 per cent. ; thus the shock effect can cause 

 but little error in the values of q and q' in the average magnet. 



Cases occur, however, in which the loss of magnetic moment is very 

 'Considerable, and the values of q and q' may have suffered in conse- 

 quence. The property seems to depend on the tempering, not on the 

 chemical character of the steel ; for various magnets which have been 

 rejected for defective retentiveness have behaved normally after further 

 treatment by the maker. 



In some of the experiments summarized in Table XII there was a 



* It is intended in future to hare four temperature cycles, discarding the results 

 of the first cycle. 



