120 



Prof. J. A. Ewing. 



longitudinal magnetising force. Other experiments with wires of 

 larger diameter and with a tube have shown that to remove the longi- 

 tudinal magnetising force when the wire is twisted generally produces 

 a very slight reduction of the circular magnetism, although in some- 

 instances it produces an actual increase (see § 22 below). In the 

 present case the breaking of the magnetising current produced no 

 effect at all comparable with that which was produced by its first 

 establishment. Re-making the current after breaking also gave 

 almost no sensible effect, 



Reversal from A to B gave a transient current of — 78, showing 

 that the circular magnetism changed to —39, and repeated reversals- 

 of the current gave the same, or very nearly the same, effect. 



§ 7. Again, if instead of twisting the wire first, and magnetising* 

 it when twisted, we apply either A or B at the zero of torsion, there 

 is no effect. Let A be kept on, and let the wire be suddenly twisted 

 to +60°: there is then a transient current equal to about +35. The 

 circular magnetisation acquired in this way is therefore somewhat 

 less than that which was reached by twisting first and magnetising 

 afterwards. If now, at +60° of torsion, we reverse from A to B, we 

 have a transient current of — 74: in other words, the circular mag- 

 netism is changed from +35 to —39. A subsequent reversal from 

 B to A gives + 78, and so on. 



§ 8. If we next keep A made, and change the torsion suddenly 

 from +60° to —60°, we get a current, not of —78, but of only about 



— 72. A twist back from —60° to + 60° gives a still smaller positive 

 deflection, and successive repetitions of the same operation, under 

 constant magnetising force, cause a gradual diminution of range, 

 until, after many back and forth twi stings, the transient currents are 



— 64, when the wire is twisted from +60° to —60°, and +64, when 

 it is twisted from —60° to +60°. This means that the circular mag- 

 netism is then changing between +32 and —32, on the assumption 

 that the effects are symmetrical on the two sides. That they were 

 very nearly so in the experiment cited I have no doubt ; there may, 

 however, have been a slight set towards the side on which the first 

 and greatest circular magnetisation occurred. If, after this steady 

 state has been reached by successive opposite torsions, the battery is 

 reversed from A to B at +60°, a transient current of —71 shows 

 that the circular magnetism is at once restored to its normal value of 

 -39. 



§ 9. The foregoing results may be stated more clearly and shortly 

 in tabular form, thus : — 



