the Investigation of Young' 's Modulus for Wires, 553 



The solenoids were wound on brass tubes, down the centre of 

 each of which passed a concentric inner tube, leaving an air 

 space of about 4 mm. all round, and the wires examined passed 

 along' the axes of these inner tubes ; this was to reduce the 

 effect of heating due to the current in the solenoid, and in 

 addition air was constantly pumped through this space. 



The current was started at practically (by a potentiometer 

 method), and rapidly increased to 2 amperes ; then reduced to 

 0, reversed, and taken up to 2 amperes again and returned to 

 0, completing a cycle. 



In general, for iron wires with only a small load, the first 

 effect was a contraction, followed by an expansion as far as the 

 current was increased (i. e. up to 2 amperes) ; on decreasing the 

 current a further lengthening took place, till a turning-point 

 was reached, and then contraction set in, and continued till 

 the current reached ; after reversal the first effect was a 

 continued contraction, then a turning-point was passed, and 

 expansion ensued till the current reached a maximum, and on 

 decreasing the current the expansion increased until the same 

 turning-point as before was reached, after which there was a 

 contraction till the current fell to 0. 



For heavy loads the effect was very different, and consisted 

 of a contraction till the current reached its maximum and an 

 expansion afterwards until the current reached 0. 



It seems somewhat strange that in the former case expan- 

 sion should result from an increase in the field, and also from 

 a decrease after the current has reached a maximum, and the 

 idea naturally occurred that it might be due to a rise in 

 temperature ; that this, however, is not the case, seems to be 

 shown by the fact that, while elongation due to heating (by 

 the solenoid) can be seen to take place slowly and uniformly, 

 this effect on the other hand is produced immediately on 

 changing the current, and stops the instant the decrease or 

 increase of current is stopped. Moreover, on stopping the 

 change of current no gradual contraction was observed, so that 

 the effect is presumably not due to heating by Foucault currents. 

 The amount of this extension did not appear to depend on the 

 rate at which the field was changed. Hard pianoforte-steel 

 showed, even with small loads, a contraction on magnetization 

 and an elongation on reducing the field, behaving in fact very 

 much as heavily loaded iron. 



The following are examples of cycles taken at random from 

 the results of about 600 observations. The load is the actual 

 load in grins, which was supported by the three No. 22 b.av.g. 

 wires ; the current at the turning-points is given in amperes 

 and the elongations are given in half-wave-lengths of sodium 

 light ; -f indicates elongation, — shows contraction. 



