in changing the Dimensions of Iron. 355 



lume of the Philosophical Magazine, Dr. Joule gives accounts 

 of numerous experiments made upon wires and bars of soft iron, 

 cast iron, soft and hardened steel, subjected to various pressures 

 and tensions while they were magnetized. As an example of 

 the effect of tension on the phenomena, he states that in the case 

 of a bar 1 foot long and | of an inch in diameter, a tensile force 

 of about 600 pounds caused all the phenomena of changes of 

 length to disappear, even with a current which produced a de- 

 flection of 58° in the needle of the tangent-galvanometer ; but 

 when a current of 61° was passed around this bar, subjected to 

 a tension of 1040 pounds, it shortened 2*8 divisions. With a 

 tension of 1680 and the same current the bar shortened 4*5 di- 

 visions. Joule, from his experiments, deduces this law; viz. 

 In the case of tension the shortening effect is 'proportional to the 

 current traversiny the coil multiplied by the magnetic intensity of 

 the bar. He further states that "it is extremely probable that 

 the shortening effects are proportional, cceteris paribus, to the 

 square root of the force of tension." 



In the case of bars of cast iron he finds that their elongation 

 is equal, if not superior, to those of soft iron when magnetized 

 to the same degree; and an increase of tension in them does 

 not produce half the retraction which is caused in soft iron bars 

 in similar circumstances. 



Bars of soft steel acted like the bars of iron ; but the superior 

 retentive powers of the former enabled him to trace better the 

 elongating effects of the permanent magnetism, which diminished 

 with the increase of tension and at last disappeared altogether ; 

 but with bars of perfectly hardened steel no sensible change in 

 their lengths was produced by charges of permanent magnetism, 

 and the temporary shortening effect of the coil was proportional 

 to the magnetism multiplied by the current traversing the coil. 

 The shortening effect did not in these cases sensibly increase 

 with the increase of tension. 



On subjecting bars of wrought and east iron and soft steel to 

 pressure, Joule found that it had no sensible effect upon the 

 extent of their elongation. A hard steel cylinder a foot long, 

 when submitted to the same experiments, with a pressure of 80 

 pounds, "suffered a diminution of length equal to 0*1 of a divi- 

 sion of the micrometer, with a current capable of giving a mag- 

 netic polarity of 1*7." 



At the termination of his paper Dr. Joule gives the following 

 "postscript," " I have already, in the former part of this paper, 

 described an experiment which indicated that no alteration in 

 the bulk of a bar of soft iron could be produced on magnetizing 

 it. I thought, however, that it would be interesting to confirm 

 the fact by an observation of the alteration of the dimensions of 



2A2 



