336 Dr. J. R. Ashworth on Magnetic Hysteresis 



ferromagnetic elements — iron, nickel, and cobalt, but con- 

 sideration o£ the relation of intensity of magnetization to- 

 field-strength was deferred. It is the object of the present 

 paper to trace the results of applying the magnetic equation 

 to the intensity (I) as a function of the field (H). 



2 In the passage of a vapour to the liquid state along 

 an isothermal, below the critical temperature and critical 

 pressure, there are three well-marked stages. In the first 

 the vapour is of small density, and, until it approaches 

 saturation, it deviates but little from the gas law ; in the 

 second there is an abrupt and very large increase of density 

 in the passage to the liquid state ; and in the third stage the 

 density of the liquid increases slowly with pressure, and 

 finally approaches a constant value at high pressures. 



Ewing has remarked that the curve of magnetization is 

 characterized by three stages. In the first there is a growth 

 of magnetism as the field increases which is small and nearly 

 proportional to the field ; in the second there is a rapid and 

 large increase of magnetic intensity for a very small change 

 of field-strength ; and in the third stage the magnetism, 

 augments very slowly and at a diminishing rate, so that in 

 strong fields the intensity becomes nearly constant. 



In fluids, as the temperature rises, the second stage' 

 becomes shorter and shorter and disappears at the critical 

 temperature ; and, similarly, in magnetism, as the tem- 

 perature increases, the second stage grows less and less and 

 vanishes at the critical temperature. Thus there is a general 

 agreement in the manner in which fluid-density increases 

 with pressure both below and above the fluid critical tem- 

 perature and the manner in which magnetic intensity 

 increases with field-strength both below and above the 

 magnetic critical temperature. 



3. The sudden change of density which takes place when- 

 a saturated vapour abruptly condenses to a liquid is not now 

 regarded a.: a discontinuity, but is treated as the passage 

 from one state to the other by a continuous path having a 

 double inflexion in it as in the figure (fig. 1, e a b c e)\. 

 Tracing this isothermal, we have the fluid in the first stage 

 from to e' as vapour ; at e' the vapour is saturated, and the 

 second stage begins with abrupt transition to e, and from e 

 to / and onwards we have the third stage. Under certain 

 conditions, particularly freedom from disturbance, the fluid 

 may follow the path from e 1 towards a, and also, on returning 

 from liquid to vapour, the fluid may continue along the path 



