260 Mr. H. Tomlinson on raising Iron under Temporary 



ratiire was reached. Other experiments were made with 

 smaller and smaller torsional stresses until only the cardboard 

 scale-pans were left to produce torsion : even here, however, 

 it was impossible to get any sign of temporary untwist on 

 heating. This mode of producing torsion was accordingly 

 abandoned, and the stress was applied in the manner described 

 in Experiment II. 



Experiment II. — A cork was fitted into the central aperture 

 of the block 0, and through the centre of the cork was passed 

 a knitting-needle about 25 centim. in length and \\ millim. 

 in diameter, which was magnetized rather feebly by rubbing 

 it with a small bar-magnet. The knitting-needle, when first 

 placed in position, was nearly in the magnetic meridian, but 

 afterwards the block B was turned until the needle eventually 

 was nearly at right angles to the plane of the magnetic meri- 

 dian, so that the wire might be under a feeble torsional stress 

 due to the action of the earth's horizontal magnetic force on 

 the needle. Immediately the wire reached a bright red tem- 

 perature there was a very perceptible sudden temporary 

 untwist, and on cooling a sudden temporary twist at nearly 

 the same temperature *. This experiment was repeated with 

 another piece of the same iron wire ; but now the needle, 

 instead of being twisted through 90 degrees from the north 

 and south position, in which it originally lay when there was 

 no torsion on the wire, was only twisted through about 10 

 degrees. Even the extremely feeble torsional stress now 

 acting on the wire was not only sufficient to produce the phe- 

 nomenon, but also a very decided permanent twist. The 

 permanent twisting began directly the wire, on being heated, 

 reached a dull red heat, and continued with increasing ra- 

 pidity until the temporary untwist occurred at a bright red 

 heat. 



Experiment III. — The temperature at which, on cooling, 

 the sudden change took place was evidently very much higher 

 than that at which it occurred with the specimens of iron used 

 by Mr. Gore and Professor Barrett; and this I felt inclined to 

 attribute to the comparative softness of the iron used by my- 

 self. I accordingly tried several other specimens of iron and 

 steel, both in the annealed and in the unannealed condition. 

 With some of these the phenomenon did not occur on cooling 

 until a dull red, or even at a still lower temperature ; but it 

 was found essential to its production that the iron should 

 have been previously raised to the temperature of bright incan- 



* These effects can easily be shown by merely heating a small portion 

 the wire with a Bunsen- burner. 



