im changing the Dimensions of Iron. 175 
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 dimin- 
ished with the increase of tension and at last disappeared al- 
together ; 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 travers- 
ing the coil. The shortening effect did not, in these cases, 
sensibly increase with the increase of tension. 
7 
when submitted to the same experiments, with a pressure of 
80 pounds, “suffered a diminution of length equal to O-l of a 
division of the micrometer, with a current capable of giving a 
ft iron bars as 
tain of the iron being in cross directions with respect to : 
Polarity in the two cases; and partly, perhaps, to the iron tube 
e 
™m. oO 
Dr. Joule says: “The law of elongation naturally ae 
Joint operation of the attractive and repulsive forces of the 
