Changing the Magnetic Moments of Steel Magnets. 295 



netic moment of one of his specimens by annealing, Mr. Gray- 

 tells me, he believes to be due to the known presence of 

 manganese in it. 



The behaviour, however, of alloys in the annealing and 

 magnetic retentiveness of steel magnets needs further eluci- 

 dation before anything very definite can be said on the subject. 

 The effects of small quantities of tungsten in increasing the 

 magnetic retentiveness of steel are well known ; whereas the 

 recent experiments of Dr. J. Hopkinson, J. T. Bottomley, 

 and others tend to show that a very moderate quantity of 

 maganese in steel almost totally destroys, not only the mag- 

 netic retentiveness, but even the magnetic susceptibility. 



With respect to the " silver steel," of which my magnets 

 are made, I am in doubt as to whether it really contains silver. 

 Some well-known steelmakers say it is only a trade name. 



In preparing the magnets, great care was taken to have 

 them made straight, and the ends made as accurately as pos- 

 sible at right angles to their length. In tempering them they 

 were put into an iron tube having one end closed, and the 

 whole put into a brisk coal fire and left there till they attained 

 a bright red heat. The tube, with the magnets inside, was 

 then taken out, and, with the open end temporarily closed by 

 a glass plate, was held vertically above a vessel of water at a 

 temperature of 15° C. and about 20 inches deep. The glass 

 cover was then quickly withdrawn, and the magnets were 

 allowed to drop perpendicularly into the water, thus making 

 them all glass-hard. 



A greater number of magnets than were actually required 

 were treated in this manner, and only the straightest and most 

 uniform in temper were chosen for the experiments. This 

 was the method employed in tempering all the glass-hard 

 magnets, and was adopted mainly in order to obtain an indi- 

 cation of what kind of results were to be expected. In sub- 

 sequent experiments, however, a method somewhat similar to 

 that used by T. Gray and Barus will be employed. The two 

 exceptions to glass-hardness already referred to (those tem- 

 pered blue and yellow) were first tempered glass-hard along 

 with the others ; they were then laid on the top of a hot 

 metallic plate, where they were allowed to remain till they 

 exhibited the oxide tints characteristic of those tempers. 



The magnets were all magnetized to saturation by placing 

 them between the poles of a large RuhmkorfT electromagnet, 

 excited by a dynamo giving a potential of between 80 and 90 

 volts. During the process of magnetizing, the magnets were 

 reversed several times between the poles of the electromagnet 

 and then finally magnetized. This was done in every case 



