14 



MB. H. TOMLINSON ON THE INFLUENCE OP STRESS AND 

 MAGNETISING solenoid not excited. 



From this last experiment it would seem that for these comparatively large 

 amplitudes the vibration-period is slightly greater for the magnetised than for the 

 unmagnetised wire, and that consequently the results previously obtained hy the 

 author with less perfect apparatus are so far confirmed. 



The Limit of Magnetic Elasticity. 



From Experiment V. it is evident that when the value of the magnetising stress 

 becomes as small as 0'104 the effect of continued reversals of it on the internal 

 friction is still perceptible. Now, as any such effect was attributed to permanent * 

 twist imparted to the molecules, first to this side and then to that, according to the 

 direction of the magnetising current, an attempt was made to ascertain how far the 

 magnetic stress might be increased from zero upwards before the molecules would be 

 permanently twisted by it. 



Experiment XIII. 



In the axis of a magnetising solenoid 11 '43 centuns. in length was placed a 

 bundle of well-annealed iron wires, consisting of twenty pieces, each 8 centims. in 

 length and '1 centim. in diameter. The axis of the magnetising solenoid was 

 coincident with that of a second solenoid, made up of 814 turns of cotton-covered 

 copper wire ; the second solenoid surrounded the first, and was connected up with a 

 very delicate THOMSON'S reflecting galvanometer. A single GROVE'S cell, with a box 

 of resistance coils in circuit, actuated the magnetising solenoid, and the magnetism 

 imparted was measured by the throw of the needle of the galvanometer produced by 

 the induced current which resulted on closing the battery circuit. The permanent 



' The word " permanent," whenever used in this memoir in connection with strain, must be taken as 

 denoting merely that the strain does not immediately disappear on the removal of the stress. The author 

 believes that such strains as have occurred in his experiments are for the most part sub-permanent. 



