Mr. G. Gore on Electrotorsion. 71 



tion induced in the bar by the coil-current, and transverse mag- 

 netism induced in it by the axial one. 



The torsions are remarkably symmetrical, and are as definitely 

 related in direction to electric currents as magnetism itself. The 

 chief law of them is — A current flowing from a north to a south 

 pole produces left-handed torsion, and a reverse one right-handed tor- 

 sion (i. e. in the direction of an ordinary screw). Although each 

 current alone will produce its own magnetic effect, sound, and in- 

 ternal molecular movement, neither alone will twist the bar, unless 

 the bar has been previously magnetized by the other. Successive 

 coil-currents alone in opposite directions will not produce torsion, 

 neither will successive and opposite axial ones. 



The torsions are influenced by previous mechanical twist in the 

 iron, by mechanical tension, and by terrestrial magnetic induction. 

 The direction of them depends both upon that of the axial and of the 

 coil-currents, but appears to be determined most by the former. A 

 few cases occur in which the currents, instead of developing torsion, 

 produce detorsion ; but only two instances, out of many hundreds, 

 have been met with in which torsion was produced in a direction 

 opposite to that required by the law. 



Single torsions vary in magnitude from 0*5 millim. to nearly 30 

 millims. of movement of the end of an index 47 centimetres long ; 

 the smaller ones occur when the two currents are transmitted 

 alternately, and the large ones when they are passed simultane- 

 ously ; the former generally leave the bar in a twisted state, the 

 latter do not. Those produced by axial currents succeeding coil 

 ones are nearly always much larger than those yielded by coil-cur- 

 rents succeeding axial ones, because the residual magnetism left by 

 the coil-current is the strongest. The order of succession of the 

 currents affects the torsions in all cases, altering their magnitudes, 

 and in some few instances even their directions. In steel all the 

 torsional effects are modified by the mechanical and magnetic 

 properties of that substance. 



Each current leaves a residuary magnetic effect in the bar, 

 amounting in iron to about one tenth of its original influence. The 

 residuary magnetism of coil-currents is affected and sometimes re- 

 versed by axial ones ; and that of axial currents is also removed by 

 coil ones, and by a red heat. The condition left by an axial current 

 is smaller in degree and less stable, in a vertical iron wire or one in 

 the terrestrial magnetic meridian, than that left by a coil one, partly 

 because of the influence of terrestrial magnetism ; but in a position 

 at right angles to that the effect is different. 



The torsion produced by a coil-current may be used as a test, 

 and partly as a measure, of the residuary effect of an axial one ; 

 and that produced by an axial current may be employed to detect, 

 and to some extent measure, ordinary magnetism in the bar. As 

 an opposite coil-current at once reverses the ordinary longitudinal 

 magnetism of a bar of iron, so also an opposite axial one at once 

 reverses its transverse magnetism. 



Many instances have been met with in which the trausverse and 



