and Retractions of Rods of Iron and Steel. 199 



same rod was subjected to a tension of 462 lbs. and a current of 

 60° 15', it behaved like my horizontally suspended steel rod 00; 

 that is, it retracted on making the circuit, but it elongated more 

 than it had previously retracted when this circuit was broken. 

 With a tension of 1680 lbs. the rod retracted and elongated by 

 equal amounts on making and breaking the circuits. In Dr. 

 Joule's experiments on a "hardened steel wire, 1 foot long, \ of 

 an inch in diameter, tension 80 lbs.," he observed no effects 

 until the current reached an intensity of 45° 40' ; then this rod 

 also elongated and retracted by equal quantities on making and 

 breaking the circuits. With tensions of 408 lbs. and of 1030 

 lbs. this rod behaved in the same manner, but the elongations 

 and retractions did not begin to show themselves with the re- 

 spective tensions until the currents had respectively reached the 

 intensities of 60° 20' and 48° 33'. Summing up these results, 

 Dr. Joule states : — " From the above experiments we find that the 

 induction of permanent magnetism produces no sensible effect on 

 the length of a bar of perfectly hardened steel, and that the 

 temporary shortening effect of the coil is proportional to the 

 magnetism multiplied by the current traversing the coil. The 

 shortening effect does not in this case sensibly increase with the 

 increase of tension." We have no reason to doubt the truth of 

 this statement when applied to rods subjected to tension ; but 

 my experiments show that when the rod 000 (of soft steel) and 

 the rod 00 (of hard steel "drawn to blue") were not subject to 

 such strains, and indeed freed as far as possible from all strain, 

 they were permanently elongated after they had received their 

 permanent magnetism,, and also that the rod (of hard steel 

 " drawn to yellow ") had a permanent retraction given to it with 

 its permanent magnetic charge. 



My experiments have been made with such conscientiousness 

 that at present I am not able to doubt the reality of these effects ; 

 but they should be repeated on fresh bars, and this I intend to 

 do at some future day. 



It is important that I should here call attention to the fact 

 that the coefficients I have given in the appended Tables are 

 derived from measures on only one rod of each species of metal ; 

 and it may be that a considerable range in the elongations and 

 retractions may be found in rods made of the same kind of iron 

 or of steel. I hope to be able to present a new series of deter- 

 minations of these constants, to be made with the apparatus 

 already described, which employs the displacement of Newton's 

 rings as a means of measuring the changes in the longitudinal 

 and transverse dimensions of the rods. 



When it is considered that the greatest motions which have 

 been the objects of my study have their existence in the space 



