204 Mr. C. A. Cams- Wilson on the Behaviour 



Hence the change induced in steel by permanent strain 

 must be due to a permanent straining of the atoms in the 

 molecule. 



We should then expect to find evidence of a straining of an 

 entirely different order and nature to that due to a molecular 

 disturbance. Such evidence is forthcoming. 



It has long been known that if a steel bar lying in a mag- 

 netic field of limited intensity be subjected to tension in the 

 direction of the magnetizing force, the magnetization of the 

 bar will be increased. 



I have made a series of experiments on this subject, and 

 find that the intensity of magnetization increases regularly 

 with the load up to the yield-point. 



The following experiments were made : — 



The bar to be tested was turned in the lathe, and a screw 

 cut on each end ; it was placed vertically in the testing- 

 machine ; to each end was screwed a nut, the top one (A) of 

 gun-metal, the lower one (B) of steel (see fig. 1, Plate IV.). 

 The former was of gun-metal so as to remove the free upper 

 extremity of the bar as far as possible from the neighbourhood 

 of large masses of magnetic material. These nuts rest on 

 steel collars, C C, which have conical surfaces to fit into coni- 

 cal holes in the main shackles, D D, of the testing-machine. 

 The bearing of the nuts on the collars is spherical, so as to 

 allow of a free movement of the specimen, and to ensure a 

 direct pull. A short distance above the upper end of the bar 

 a small magnet, E, is hung by a silk fibre, the suspension of 

 which is rigidly attached to the upper shackle, so that any 

 small motion of the latter involves the same motion of the 

 needle ; a silvered mirror is attached to the needle, which 

 reflects a beam of light from a lamp, F, onto a horizontal scale, 

 G ; H is a controlling-magnet. 



A small displacement is given to the magnet (i.) when 

 there is no load on the bar, and the time of 20 complete 

 oscillations observed ; (ii.) the same with a small load on; 

 (iii.) the load is then removed, and the time of 20 oscilla- 

 tions again observed. The results are recorded in the accom- 

 panying Table. 



Experiment I. — Bar of soft steel, never loaded before : cross 

 sectional area 0*306 square inch, 19*5 inches long; elonga- 

 tions measured on 10" by a vernier and microscope reading 

 to thousandths of an inch. 



An automatic stress-strain diagram was taken (see curve A, 

 Plate IV.). The time of 20 oscillations of the needle was first 

 observed with the .bar out — 102*6 seconds. From this was 

 deduced a?, the square of the number of oscillations per 

 minute, and this was deducted from N 2 , the same when the bar 



