624 CONSTANTS OF MAGNETISATION. 



the intensity of magnetisation of the earth, regarded as a sphere 

 uniformly magnetised, is equal to the fraction of its action at 



47T 



the equator (355), which gives 



I a = 0-33092 = 0-079. 

 4?r 



Gauss found in like manner that the magnetic moment of a 

 steel bar weighing a pound was 10087-7, his numbers being re- 

 duced to C.G.S. units. Assuming that the density of the steel is 

 7 -8, and that the pound in question is 453*6 grammes, the mean 

 intensity of magnetisation will be 174, or 2200 times the terrestrial 

 magnetisation. 



Magnets such as are used in laboratories or for observations 

 on terrestrial magnetism, have often a mean magnetisation of 200 

 to 400 units. The maximum magnetisation, however, which a 

 magnet can acquire depends not only on the nature of the steel 

 and its mode of tempering, but also on the dimensions of the bar. 

 It is greater as the bar has the form of a longer and thinner cylinder. 



Magnetisation, in fact, is especially localised in the superficial 

 layers, either because. the methods of magnetisation used do not 

 readily act on the internal parts or because the effects of the tem- 

 pering have not themselves penetrated to a great depth. Further, 

 the reaction of a magnet on itself, or the demagnetising force (407), 

 is less as the bar is longer in the direction of magnetisation. 



By working on very thin rods, Kohlrausch* has observed that the 

 magnetic moment of steel may attain 100 units per gramme, which 

 would give a strength of magnetisation of 780, or nearly 10,000 

 times that of the earth. 



1197. METHOD OF DETACHMENT. The intensity of magnetisa- 

 tion may, in certain cases, be determined directly, without having 

 recourse to the measurement of a magnetic moment. 



Let us assume that, in a cylinder uniformly magnetised, a section 

 S has been cut through at right angles to the axis, and that the 

 surfaces have been brought in contact. The magnetic density on 

 the surfaces is equal to the intensity of magnetisation l a (322) ; 

 the action for unit surface is equal to 27712, as for electrified sur- 

 faces (41) ; and the total attraction of the two surfaces is equal 



* KOHLRAUSCH. Leitfaden der Prakt. Physik, Fourth Edition, p. 174. 1880. 



