16 



M]R. H. TOMLINSON ON THE INFLUENCE OF STRESS AND 



the following experiment shows that even when the length is so great as to render 

 such an objection quite out of the question there is an apparent limit of magnetic 

 elasticity. 



Experiment XIV. 



A magnetising solenoid, no less than 354'3 centimetres in length, was prepared, 

 having coiled on it, in a single layer, silk-covered copper wire ; the number of turns 

 per centimetre was 8'5. The secondary coil consisted of 5909 turns of silk-covered 

 copper wire, coated with shellac varnish, laid on in six layers ; the resistance of 

 this coil was nearly equal to that of the ballistic galvanometer. The bundle of iron 

 wires was increased in number to 40, and the length of each was 325 centimetres, so 

 that, regarding the bundle as one solid rod, the length of it was 512'5 tunes the 

 diameter. The mode of experimenting was the same as before. 



In reading the deflections there was certainly no error equal to '5 of a division, and 

 the first trace of the molecules taking a permanent set is when the magnetic stress 

 has reached 0'03667, or about one-fifth of the value of the horizontal component of the 

 earth's magnetic force. The first decided permanent set does not apparently occur till 

 the stress has reached 0'04821, a value very close to that recorded in the last experi- 

 ment as the limit of magnetic elasticity. 



A similar experiment to the above was tried with a wrought-iron rod in an unan- 

 nealed condition, and in this case there was not the slightest trace of permanent set 

 until a stress of 0'0482l was reached, whilst the deflection due to temporary magneti- 

 sation with this stress was 538 scale-divisions. 



These last experiments seem to show that between the values and ^H of magnetic 

 stress there is no permanent set whatever ; but in the next experiment it will appear 

 that, if we operate by the method of reversals, permanent set may be detected with 

 much smaller stresses. 



