produced in Iron Wires by Magnetization. 361 



and must be added to the ordinates of the curves plotted on 

 fig. 5. It is impossible to say how these curves would be 

 affected by the correction, but probably the correction in- 

 creases with the intensity of magnetization. 



Changes in the length of the wire due to variations in 

 temperature are another possible source of error. These 

 changes take place more slowly than the elongation produced 

 by the magnetization, so that the two effects may be separated. 

 To retard them still more the core of the solenoid was made of 

 two co-axal cylinders and the space between them filled with 

 water. With this arrangement the current could be applied, 

 the reading taken, and the current shut off before the gradual 

 change of the zero point due to the heating of the wire by the 

 current could be observed. Changes in the temperature of 

 the room when they did occur were too slow to noticeably 

 affect the results. 



Summary. 



The following is a summary of the results obtained by 

 this investigation : — 



1. A relation has been obtained between the elongation 

 due to magnetization and the intensity of magnetization of 

 soft iron. When the elongation has been corrected for the 

 contraction caused by the ~B 2 /8tt force, the relation may be 

 expressed thus : — A rod of iron elongates slowly until an 

 intensity of about 800 is reached. After that point the rod 

 elongates more rapidly and attains a maximum value of about 

 60 x 10 -7 part of its length at an intensity of 1200. With 

 greater intensities the elongation diminishes, the curve being 

 approximately a nearly vertical straight line. 



2. Hardening the wire diminishes the corrected elongation, 

 and the wire attains its maximum length in a less intense 

 field. 



3. It has been shown that when the wire is stretched, the 

 change in elasticity due to the magnetization produces an 

 important effect. Neglecting this correction, the effects of 

 stretching are similar to those caused by hardening the wire. 



Before concluding I would acknowledge my indebtedness 

 to Professor Rowland, not only for suggesting the investiga- 

 tion to me, but also for his assistance and kind consideration 

 afterwards. 



I also owe many thanks to Dr. Ames and to Dr. Duncan 

 for advice and help. 



