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XVII. The Effects of Tension and Quality of the Metal upon 

 the Changes in Length produced in Iron Wires by Mag- 

 netization. By Byron Briggs Brackett *. 



ALL the specimens of iron wire experimented upon were 

 about 2 metres long and 1'25 millira. in diameter. 

 While under test a wire was suspended vertically so that 

 tension could be applied by weights attached to its lower end. 

 About the central portion of the wire was placed the mag- 

 netizing solenoid, 84 centim. long and with a free internal 

 diameter of 3'6 centim., allowing no contact between it and 

 the wire or parts of the apparatus attached to the wire. The 

 variations in the length of 70 centim. of the wire within the 

 solenoid was then observed by means of a mechanism, em- 

 ploying a lever and tilting mirror, in such a way as to magnify 

 the actual changes about 25,000 times, and hence to give 



dl 

 readings for y multiplied by 1,750,000. 



For all tests made, with one exception, the changes in 

 length caused by a definite magnetizing field were read both 

 with the field on and after it had been removed. These 

 readings were made for gradually increasing fields up to the 

 limits of the apparatus, the points taken on each curve being- 

 very numerous and especially close together wherever the 

 curve changes its direction rapidly. 



All of these tests were followed by careful tests for the mag- 

 netic induction in the specimens under conditions identical 

 with those of the elongation tests. From these, the induction 

 in the wire is known for each elongation, and curves are 

 plotted between elongation and induction, as well as between 

 elongation and magnetizing field. 



Three different kinds of wire were tested : piano-wire in 

 its natural condition : annealed piano-wire : and very soft 

 annealed iron wire. 



Results obtained : — 



1. Natural Piano-ioire. 



This wire could not be kept straight without considerable 

 tension ; but under the tensions employed (658 and 1949 kg. 

 per sq. cm.) no change of length was observed until the 

 fields were reached at which the magnetization rises rapidly. 

 Chanses in leneth were first observed when the field was 



* From the Johns Hopkins University Circulars, June 1897. 



