16* 



On the anomalous 'Ulagnetie Action of Hot Iron between the 



and Blood-red Heat. By Peter Barlow, Esq. of the Royal Mili- 

 tary Academy. Communicated by Major Thomas Colby, of the Royal 

 Engineers, F.R.S. Read January 24, 1822. [Phil. Trans. 1822, 



p.m.-] 



Finding the attractive power of soft malleable iron and steel for a 

 magnet greater than that of cast-iron and hard steel, the author was 

 desirous of ascertaining the effect of heating these bodies in a fur- 

 nace, so as to render them perfectly soft, upon their magnetic power. 

 With this view the bars were rendered white-hot, and being placed 

 in the direction of the dip, their powers were found nearly equal. It 

 was however found that there was a point between the white heat, 

 at which all magnetic action was lost, and the blood-red heat, at 

 which it was strongest, at which the iron attracted the needle the 

 contrary way to which it did when cold; viz. if the bar and com- 

 pass were so placed that the north end of the needle was drawn to it 

 when cold, the south end was attracted during the interval above- 

 mentioned. 



The author then proceeds to detail some further experiments in 

 illustration of this anomalous magnetic action, from which it appears 

 that the quantity of magnetic attraction at a red heat is influenced 

 by the height or depth of the centre of the bar from the compass ; 

 and as the natural effect of the cold iron was changed by placing the 

 compass below the centre of the bar, it became a question how far 

 the negative attraction was also changed. To decide, the compass 

 was lowered to within six inches of the bottom of the bar, when the 

 cold iron produced a deviation of 21, by attracting the south end of 

 the needle. At a white heat its power ceased ; but as this subsided 

 to bright red the negative attraction amounted to 10^, the north end 

 of the needle being attracted to the iron ; it then gradually returned 

 to due north, and ultimately to 70 30' on the opposite side. 



Mr. Barlow then gives the results of another series of experiments 

 made with the bars inclined in the direction of the dipping-needle, 

 showing that where the negative attraction was greatest the natural 

 attraction was least, that is, opposite the middle of the bar, or in the 

 place of no attraction. 



Being doubtful how far the heat itself, independent of the iron, 

 might be the cause of the anomalous action above described, the 

 author substituted a heated copper bar for that of iron, but it pro- 

 duced no motion in the needle. He thinks it probable that the 

 anomalies may depend upon the iron cooling faster towards its ex- 

 tremities than towards its centre, one part of the bar thus becoming 

 magnetic before the other, and choosing a different species of at- 

 traction. 



