PROPERTIES OF IRON. 629 



As the method of rings only gives variations of the magnetic 

 state, we only know the temporary magnetism by induced discharges 

 which correspond to the making or breaking of the principal current, 

 and the residual magnetism is determined by the difference of dis- 

 charges in the making and breaking of the current. 



The ring is thus in each experiment in a state which is im- 

 perfectly known. We should limit ourselves to only employing 

 increasing currents, and the apparatus is out of use when the 

 strongest currents have passed. 



1203. PROPERTIES OF IRON. The first numerical results on 

 the magnetisation of soft iron by currents are due to Lenz and 

 Jacobi.* They used the method of induction with cylindrical 

 bars, and concluded from their experiments that the magnetisation 

 is proportional to the intensity of the current. About the same 

 time Joule f demonstrated the capital fact of the existence of a 

 maximum. He worked by detachment, ascertaining what weight 

 was necessary to separate two cylinders placed end to end and 

 magnetised by a current, or two segments of a ring along a plane 

 parallel to the axis. Joule observed that the weight commences to 

 vary proportionally with the square of the intensity, that is to say, 

 the magnetising force, in conformity with the law of Lenz and Jacobi, 

 but then much less rapidly, and tends lastly towards a maximum. 

 In the following table, taken from his memoir, the first four num- 

 bers relate to his own experiments ; the three following are deduced 

 from the previous experiments of Nesbit, of Henry, and of Sturgeon : 



Field 



21,880 

 20,8lO 

 21,860 

 16,750 

 23,390 



18,120 

 1-26 17,93! *67i 20,990 



The intensity of magnetisation will then be represented as a 

 function of the intensity of the current or of the magnetising force 



* LENZ and JACOBI. Pogg. Ann., Vol. XLVIII., p. 225. 1839. 



f JOULE. Annals of Electricity, Vol. IV., p. 474, 1839; Vol. v., p. 187, 1840. 

 Scientific Papers, Vol. I., pp. 15-38. 



