of Steel under Mechanical Stress, 205 



was in place. The results are plotted on fig. 1, Plate V. as two 

 curves, the upper one, A D being the curve of temporarily- 

 induced magnetism, with load on, and the lower one, A 2 , that 

 of permanent magnetism, with load off. The arrow shows 

 the yield-point. The two curves cross one another twice, and 

 coincide in a third point. The third curve, A 3 , is the curve 

 of extensions from the second column of the Table, 186 times 

 full size. Curve A 4 is the curve of permanent set, same scale 

 as A 3 . This bar was broken. The last observation was after 

 the fracture. 



Experiment II. — Bar of soft steel never loaded before : 

 cross- sectional area 0*308 square inch, 19 inches long; diagram 

 measures elongations of 10"; square of number of oscilla- 

 tions per minute with bar out 121 ; ditto, bar in, no load on, 

 1849. The results are plotted as a curve in fig. 2, Plate V. 

 The arrow shows the yield-point. The observations at this 

 point were taken after the set. The stress-strain diagram is 

 shown at B, Plate IV. This bar was broken. 



Experiment III. — Wrought-iron bar never loaded before : 

 cross-sectional area 0203 square inch. The extensions on 

 the diagram, C, Plate IV., are on a length of six inches. The 

 results are recorded in the Table, and plotted on fig. 5, PlatellL, 

 the ordinates here representing simply the square of the num- 

 ber of oscillations per minute. The arrow shows the yield- 

 point. The curves intersect twice. This bar was not broken. 



The circles show the loads at which observations were made. 



From the curves on Plates III. and V., it is evident that 

 the magnetic induction is increased in a regular way by 

 increased stress, and that the effect is largely permanent ; a 

 point is reached, before the yield-point, where the effect 

 produced is wholly permanent. The two curves continue 

 side by side up to the yield-point, when the large permanent 

 set which then takes place completely upsets the former con- 

 dition of the bar, as might have been expected. The temporary 

 effect is roughly proportional to the elastic extension below 

 the yield-point. 



The conclusions to which I am drawn by these experiments 

 are : — 



(i.) Mechanical straining produces an "atomic disturb- 

 ance " in a bar W'hich increases regularly with the stress. 



(ii.) For small stresses the disturbance is only partly per- 

 manent, but as the yield-point is neared it becomes wholly 

 permanent. 



(iii.) The magnetic properties of a loaded bar are in general 

 different from those of the same bar unloaded, but there is a. 

 certain stress or range of stresses over which the bar has the 

 same magnetic properties whether the load be on or off. 



Phil, Mag. S. 5. Vol. 29. No. 177. Feb. 1890. Q 



