in changing the Dimensions of Iron. 

 Experiment I. 



353 



Deflection 

 of galvano- 

 meter. 



Tangent of 

 deflection. 



Elongation 

 or shorten- 

 ing of bar. 



Total 

 elongation. 



Magnetic 

 intensity 

 of bar. 



Square of mag- 

 netic intensity 

 divided by total 

 elongation. 



O / 



- 7 20 



128 



1-0 E. 



10 



-0-49 



240 











0-3 S. 



0-7 



-0-42 



252 



- 9 30 



167 



2-9 E. 



3-6 



-0-93 



240 











1-2 S. 



24 



-074 



228 



-14 48 



264 



5-9 E. 



83 



— 1-42 



243 











3 8 S. 



4-5 



-1-00 



222 



-23 10 



428 



10-3 E. 



14 8 



-1-87 



236 











7-6 S. 



72 



-1-26 



220 



-47 25 



1088 



16-1 E. 



23-3 



-2 22 



211 











13-9 S. 



9-4 



-1-35 



194 



-58 50 



1653 



14-8 E. 



242 



-2-21 



202 











13-3 S. 



10-9 



-1-35 



168" 



Dr. Joule now reversed the current in the helix and found 

 that a current which deflected the needle 6° 15' shortened the 

 bar 3'4 div., and that after the current was broken its magnetic 

 intensity was found reduced from —1*3 (the permanent inten- 

 sity previously given by 47° 25', see preceding Table) to —'17. 

 He then passed a current of 9° 55'; and this he found was suffi- 

 cient, not only to remove the former minus polarity of the bar, 

 but also to give it a permanent polarity of + *25, and yet to 

 leave the bar with 6*6 of the elongation belonging to its previous 

 minus polarity. 



Taking Joule's observations while the current was passing 

 around the bar, we have for the current of 6° 15' a magnetic 

 intensity of —0*12, and for the current of 9° 15' a plus magnetic 

 polarity of 0'57. We call attention to these results because 

 subsequent experimenters* seem to be unaware of these obser- 

 vations of Dr. Joule, who here first shows that a feeble current 

 will demagnetize and even reverse the polarity of a bar which has 

 previously required a far more powerful current to give it its 

 permanent magnetic charge. In the experiment given above, 

 the ratio of the current-intensities of permanent magnetization 

 and of demagnetization is 1088 to 175. 



Dr. Joule now successively replaced the above bar by two 

 others, and obtained with them similar results. He then de- 

 duces the following important law : — " From the last column of 

 each of the preceding Tables we may, I think, safely infer that 



* Wiedemann, Pogg. Ann. vol. c. p. 235 ; also R. W. Wilson, " Demag- 

 netization of Electromagnets," American Journal of Science, vol. iii. 3rd 

 Series, p. 346. 



Phil. Mag. S. 4 Vol. 45. No. 301. Mag 1873. 2 A 



