Changes in Length, in Iron Wires by Magnetization. 123 



removed. This was an elongation, and the length of the wire 

 with the field off continued to increase up to the field 

 strengths at which the induction curve turns towards the 

 horizontal. From that point on, the wires were permanently 



elongated with the field off. But this value of y did not 



exceed 3 - 5xl0~ 7 ,and could have been produced by the 

 tension caused by adding about 10 grams to the weight 

 already on the wire. At a very short distance beyond the 

 point where the wire begins to lengthen as the current is 

 broken, it shows its first change of length with the field on 

 by beginning to contract. From this point on to the end, 

 the wire contracts with the field on, and the contraction 

 increases apparently in direct proportion to the field until at 



the end, for a field of 300, y equals about 30 x 10~ 7 . 



The curves for the different tensions differ but little. 



2. Annealed Piano-wire. 



After the piano-wires had been tested in their natural 

 condition they were softened by annealing, and the same 

 wires were tested again. They then showed elongations with 

 the fields both on and off up to the point where the induction 

 curve turns towards the horizontal. The elongations here 

 are greater with the field off than when it is on. Bat on 

 each curve the elongation increases nearly in direct propor- 

 tion to the induction. Not far from the turning point of the 

 induction curve, the elongations with the field off cease to 



show any change, this value of -j not exceeding 7 x 10 -7 . 



At about the same point the curve with field on begins to 

 descend along a nearly straight line, just as it did before the 

 annealing. But here the contraction is more rapid, the slope 

 for the straight part of the curves, as plotted to field, being 

 nearly 50 per cent, greater than before annealing. 



Increased tension does not affect the general shape of the 

 curve. But it diminishes the amount of the elongation and 

 increases the final contraction with field on, while it causes 

 both elongation and contraction to begin at weaker fields. 



3. Soft Annealed Wire. 



The curves for the very soft wire are in general exactly 

 like those for the annealed piano-wire, except that the 

 changes of length are very much greater, and in some cases 



