of Magnetic Moments in Absolute Measure. 327 



had been again heated in oil to any temperature up to 310 

 Cent. , and afterwards allowed to cool slowly in air. By the 

 second magnetization, however, the difference between the 

 magnetic moments per gramme of the glass-hard and softer 

 magnets, though still in favour of the latter, was greatly dimi- 

 nished, which seemed to indicate that the greater strength of 

 the softer magnets after the first magnetization was rather 

 due to their being more easily magnetized to saturation. 



Magnets made of steel which had been heated to redness and 

 then cooled in oil had a comparatively small magnetic moment 

 when they had been cooled in cold oil ; but the magnetic 

 moments gradually increased as the temperature of the oil was 

 raised till it reached about 150° Cent., after which the mag- 

 netic moments were smaller the higher the temperature of the 

 oil. This result was not altered by the second magnetiza- 

 tion. The magnetic moments of these bars varied from about 

 60 to 80 per gramme. 



Some magnets supplied to Mr. James White, Glasgow, 

 to be used as adjusting magnets for Sir William Thomson's 

 compass, were found to have, when magnetized by a powerful 

 Ruhmkorff coil, an average magnetic moment of about 50 

 per gramme. Each bar weighed about 170 grammes, and 

 was 30 centimetres long. 



These magnets, when supplied by the maker, were com- 

 paratively soft ; and their magnetic moments were slightly 

 diminished by tempering them glass-hard and magnetizing 

 them. 



A series of experiments was made with magnets of the 

 homogeneous iron or steel supplied by Webster and Horsfall 

 for the sheathing of the 1865 cable. Each of these magnets 

 was five centimetres long, and weighed 2*27 grammes. 

 Their magnetic moments per gramme for the different tem- 

 pers were as follows : — 



Glass-hard 20*22 



Yellow 17-18 



Blue 11-29 



As supplied 12-09 



This shows a marked difference as to magnetic moment 

 between magnets made of this steel and the magnets used in 

 the former experiments. 



Thus the glass-hard magnets of the soft steel had a mag- 

 netic moment of 74*3 per gramme, while the magnetic 

 moments at the other tempers were a little greater. On the 

 other hand, as the Table above shows, the magnetic moment of 

 the glass-hard magnets made of Webster and HorsfalFs steel 



