232 Mr. J. P. Joule 0)j the Effects of Mag7ietism, S^c. 



Experiment 14. Same wire with — O"^ permanent magnet- 

 ism to begin with. Tension 408 lbs. 



In tables 13 and 14 we notice the rapid decline of the effect 

 of elongation, until at last, in table 15, with a tension of 

 740 lbs. it ceases altogether, and the new condition of shorten- 

 ing commences. With a tension of about 600 lbs. the effects 

 on the dimensions of the wire would cease altogether in the 

 limits of the electrical currents employed in the above experi- 

 ments. From the last column of table 15, which consists of 

 the columns 2 and 5 multiplied together and divided by co- 

 lumn 4, we gather that the shortening effect is very nearly 

 proportional to the magnetism of the wire into the current 

 traversing the coil. The law of the square of the magnetism 

 will still indeed hold good where the iron is sufficiently below 



