XIV. On the anomalous magnetic action of hot iron between the white and blood-red heat

Abstract

In consequence of certain theoretical results relative to the magnetic action of iron, obtained by Mr. Charles Bonnycastle, I was desirous of ascertaining the relative attraction which different species of iron and steel had for the magnet; and with this view I procured two bars of each of the following descriptions of metal, 24 inches in length, and 1 inch and a quarter square, which being placed successively in the direction of the dip, at a certain distance from the compass, the disturbance occasioned by each was carefully noted; first with one end upwards, and then with the other; and assuming the tangents of the angles as the measure of the disturbing power, I obtained the following specific results, viz. Mag. Pow. Mag. Pow. Malleable iron - - 100 | Shear steel soft - - 66 Cast iron - - - 48 | ------------- hard - - 53 Blistered steel soft - 67 | Cast steel soft - - 74 ---------------- hard - 53 | ------------ hard - - 49 As it was obvious from these experiments, that the softer the iron the greater was its power, and the contrary, I was desirous of determining how nearly these different kinds of metal would approximate towards each other in their magnetic action, when rendered perfectly soft by being heated in a furnace. With this view, bars of equal size of cast iron, malleable iron, shear steel, &c. were rendered white hot, and being placed in the direction of the dip, as before, their powers, as was anticipated, agreed nearly with each other; but still the cast iron, which was weakest while the metal was cold, exceeded a little in power all the others when hot, and the malleable iron which had the greatest power cold, had the least when hot; but the difference was not very great, and might probably arise from some accidental circumstance. While carrying on these experiments, it had been observed, both by Mr. Bonnycastle and myself, that between the white heat of the metal, when all magnetic action was lost, and the blood-red heat, at which it was the strongest, there was an intermediate state in which the iron attracted the needle the contrary way to what it did when it was cold, viz. if the bar and compass were so situated that the north end of the needle was drawn towards it when cold, the south end was attracted during the interval above alluded to, or while the iron was passing through the shades of colour denoted by the workman the bright red and red heat.