in changing the dimensions of Iron and Steel bars.. 108 
sion of 408 tbs. and of 1080 tbs., this rod behaved in the same 
manner, but the elongations and _retractions did not begin to 
show themselves with the respective 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 magnet- 
ism produces no sensible effect on the length of a bar of perfect- 
ly 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 the tension.” We have no 
reason to doubt the trath of this statement when applied to 
rods subject to tension, but my experiments show that when 
as far as possible, from all strain, they were permanently elon- 
gated after they had received their permanent magnetism ; and 
cnange. 
My experiments have been made with such conscientious- 
ness, 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. 
retractions may be found in rods made of the same kind of iron 
or of steel. 1 hope to be able to present a new series of deter- 
already described, which employs the displacements of Newton's 
rings as a means of measuring the changes in the longitudinal 
hen it is considered that the greatest motions, which have 
been the objects of my study, have their existence 1n the space 
