120 Mr. S. Bidwell. On the Changes produced by 



turbed, the sub-permanent magnetism will remain without material 

 diminution for perhaps half an hour, but in a short time it rapidly 

 falls off. If, however, the rod is shaken, or even if it is removed 

 from the coil with the greatest care, the magnetism which I have 

 called sub-permanent instantly disappears. But, after the destruction 

 of the sub-permanent magnetism, there still remains a small quantity 

 of magnetism of a nature which may properly be called permanent, 

 since it persists for days, or perhaps indefinitely, unless violent 

 measures are resorted to for its removal. I find, by experiments 

 which will be referred to hereafter, that when an iron rod has 

 once been sub-permanently magnetised by a strong current, the 

 intensity of the sub-permanent magnetisation is absolutely unaffected 

 by the action of currents weaker, or not stronger, than the first. For 

 a limited time (say half an hour) currents of varying strength may be 

 passed through the coil, and the additional magnetism produced by 

 their action is of a 'purely temporary nature, disappearing completely 

 when the current ceases to flow, and leaving the sub-permanent 

 magnetism exactly where it was before. The elongations referred to 

 in my tables and curves are due to purely temporary magnetisation. 



The strength of the magnetising current was varied by means of a 

 box of resistance coils, and was measured by a Helmholtz tangent 

 galvanometer with four separate coils, of Elliott's manufacture. After 

 the first two or three preliminary experiments, no attempt was made 

 to read the galvanometer at the time when the observations of the 

 elongations were made : for, in order to do so, it was found necessary 

 to keep the circuit closed for a period which was sufficiently long to 

 cause the coil to become heated, and confusion was introduced owing 

 to the heat expansion of the rod. A note was made of the resistances 

 successively inserted, and the currents corresponding to the several 

 resistances were afterwards leisurely and carefully determined. It 

 was soon found that the action of the battery was so constant that 

 several elongation experiments might be made on the assumption that 

 the same currents accompanied the same resistances without any 

 sensible error, except perhaps a slight one in the case of the strongest 

 currents ; but the estimated currents were from time to time checked 

 by reference to the galvanometer, and when any material variation 

 was observed, a fresh series of galvanometer readings was made. 



An examination of the three iron curves discloses the following 

 facts : — In every case the form of the curve for the first part of the 

 ascent is sufficiently nearly parabolic in form to afford confirmation 

 of Joule's law, that the elongation varies up to a certain point as the 

 square of the magnetisation. After passing the maximum, the curve 

 assumes a form which is apparently intended to be a straight line ; at 

 all events, no single observation deviates from the straight line by an 

 amount equivalent to more than half a scale division. If this is so, 



