294 Mr. W. Brown on the Effects of Percussion in 



In one of these papers the authors treat incidentally of 

 magnetic retentiveness, but more with respect to the effects 

 of annealing than of direct percussion. Annealing appears to 

 play an important part in the ultimate retentive power of 

 magnets, and on the constancy of their magnetic moments. 

 My own experiments on annealed magnets are not yet com- 

 plete enough to be put into presentable form, and with two 

 exceptions the results given are for magnets tempered glass- 

 hard. Regarding the effects of annealing generally, the 

 results of Barus, Strouhal, and Gray appear to agree on the 

 whole. 



On pages 326 & 327 of T. Gray's paper it appears that an- 

 nealing increased the magnetic moment. This is the result 

 stated generally in the text ; but a marginal note and curve 

 in the copy in my possession show that the magnetic moment 

 did not increase continuously as the annealing went on, but 

 increased at first, then diminished, then again increased — 

 thus passing a maximum when the annealing temperature was 

 about 150° C, and a minimum when the temperature was 

 about 230° C. 



Another set of results given in this paper is a series of 

 magnetic moments for bars of the same steel tempered in oil, 

 the temperature of which was varied so that the bars should 

 be suddenly cooled only to the same temperature as that to 

 which in the first set they were heated in the annealing pro- 

 cess. The results in this second set show precisely the same 

 characteristics as those of the first set, only that the maximum 

 and minimum points are much more pronounced. 



The curves given by Mr. T. Gray indicate an interesting 

 peculiarity which does not seem to have been noticed by 

 any previous observer, inasmuch as they show that the 

 minimum point may be preceded by a maximum, the effects 

 of annealing depending greatly on the kind of steel used. 

 The steel used by Gray was almost a pure charcoal steel, 

 whereas that used by Barus and Strouhal was of the kind 

 known as " English silver steel ; " that is to say, of a kind 

 similar to that which I have been experimenting on. 



Mr. Gray in his paper refers very briefly to some experi- 

 ments on other steels, chiefly, it would appear, for the purpose 

 of showing how very different results may be expected from 

 different specimens. In one set, which took an average mag- 

 netic moment of about 50 per gramme, the magnetic moment 

 was slightly diminished by hardening ; whilst in another set 

 the direct opposite was arrived at in a very marked manner, 

 showing how very much the effect of annealing depends on 

 metallic impurities in the steel. The diminution of the mag- 



