MAGNETIC PERMEABILITY OF NICKEL AND COBALT 73 



In all these the upper signs are to be taken for all values of >, less than 



, and the lower signs for greater values. 

 t> 



On applying these formulas to the observations, I have found that the 

 corrections will in no way influence my conclusions, being always very 

 small; but at the same time the calculation shows that it would be well 



R 



to diminish the ratio as much as possible. In all my rings this ratio 

 a 



did not depart very much from - ; but I would advise future experi- 



o'o 



menters to take it at least as small as ^: the amount of correction 



R 



will be very nearly proportional to the square of . 



ct 



Summary. 



The following laws have been established entirely by my own experi- 

 ments, though in that part of (2) which refers to iron I have been 

 anticipated in the publication by Dr. Stoletow (Phil. Mag. Jan. 1873). 

 When any measurements are given, they are on the metre, gramme, 

 second system. 



(1) Iron, nickel, and cobalt, in their magnetic properties at ordinary 

 temperatures, differ from each other only in the quantity of those 

 properties and not in the quality. 



(2) As the magnetizing-force is increased from upwards, the resist- 

 ance of iron, nickel, and cobalt to magnetization decreases until a 

 minimum is reached, and after that increases indefinitely. This mini- 

 mum is reached when the metal has attained a magnetization of from 

 24 to -38 of the maximum of magnetization of the given metal. 



(3) The curve showing the relation between the magnetization and 

 the magnetic permeability, or Neumann's coefficient, is of such a form 

 that a diameter can be drawn bisecting chords parallel to the axis of 33, 

 and is of very nearly the form given by the equation 



where B, &, and D are constants, jut is the ratio of the magnetization to 

 the magnetizing-force in an infinitely long bar, and 33 is the amount 

 of magnetization. 



(4) If a metal is permanently magnetized, its resistance to change of 

 magnetism is greater for low magnetizing-powers than when it is in the 

 normal state, but is the same for high magnetizing-powers. This 



