1881.] Permanent Molecular Torsion of Conducting Wires. 27 



upon giving a torsion to the wire in one direction, I find the inductive 

 force increase from 40 to 90, but in the other direction it is brought to 

 zero, and the amount of torsion some 35° required to bring the wire 

 again to zero represents exactly the twist or strain that had been 

 produced instantaneously by the passage of an electric current. If I 

 repeat the experiment, but reverse the battery current sent through 

 the wire, I find an opposite twist of exactly the same value as pre- 

 viously, and that it now requires an opposite torsion to again bring 

 the wire to zero. It is not necessary, however, to put on an equal 

 opposite torsion on wire to bring the currents to zero, for, as I have 

 shown in my late paper, the sonometer not only allows us to measure 

 the force and indicate its direction, but allows us to oppose an equal 

 electric current of opposite name, thus producing an electrical zero in 

 place of the mechanical one produced by torsion. 



Evidently here there has been a sudden change in the structure of 

 the wire, and it is a twist which we can both measure and reproduce. 

 The question a/t once becomes, has a molar twist been given to the 

 wire such as would be detected by the arm or free end of the wire, 

 or a molecular change leaving no trace upon its external form of what 

 has passed ? 



It will be found that, notwithstanding that it requires some 40° 

 of torsion to annul the effects of a passage of an electric current, no 

 visible movement nor any tendency of the free end to turn in the 

 direction of the twist it has received can be observed. I believe, 

 however, to have noticed a slight tremor or movement of half a 

 degree, but as I could not always reproduce it, and as it is so slight 

 compared with the 40° of internal twist, I have not taken it into 

 account, for if the wire is firmly fastened at both ends, no molar 

 torsion being possible, except an elastic one, which would instantly 

 spring back to zero, the current on passing produces its full effects of 

 twist and it is permanent. Thus, the molecules have in some extra- 

 ordinary way rearranged themselves into a permanent twist, without 

 the slightest external indication of so great a change having taken 

 place. An equally remarkable change takes place in aid of, or against 

 (according to direction of current) an elastic permanent strain. 

 Thus, if I first put the wire under 40° right-handed permanent tor- 

 sion, I find its value to be 50. Now, passing the positive of battery 

 through its free end, and negative to fixed end, the induction currents 

 rise at once in value to 90 ; if, now, the negative is momentarily 

 passed through the free end and positive to fixed end the induced 

 currents at once fall to 10, and these effects remain, for on taking off 

 the elastic torsion the wire no longer comes to zero, but has the full 

 twist value produced by the current. 



Tempered steel gave only one or two degrees against 50 for soft 

 iron, but supposing this might be due to its molecular rigidity, I care- 



