On the Rotation cawed by Electricity developed by Friction. 495 



This expression includes the ordinary formulae for the centre 

 of parallel forces ; for if we put foi- u its value oca. + ?//3 + zj, and 

 similar values for «', u", &c., we Hud 



SXm IXx SXy XXz 



2X "^ SX*"^ 2X ^+ 2X '^' 

 whence the coordinates of the point of application of the result- 

 ant are manifestly those given by the ordinaiy formulae for the 

 centre of parallel forces. 

 King's College, 

 May 7, 1851. 



[To be continued.] 



LXX. On the Rotation caused by Electiicity developed by Friction. 

 By E. r. August*. 



WHEN a freely suspended magnetic needle is brought near 

 the revolving disc of an electric machine the needle vi- 

 brates, and after a few oscillations to and fro commences to rotate. 

 Instead of the magnetic needle, a needle of any conducting material 

 may be used with the same result. The rotary motion of the needle 

 is opposed to that of the disc, that is to say, the point of the 

 needle moves in a direction opposite to that of the adjacent par- 

 ticles of the disc. Hence the rotation above the axis is the re- 

 verse of that below it. At the same height as the axis of the 

 disc no rotation occurs at either side of it, and when the needle 

 on being raised upward passes this position, its previous mo- 

 tion is arrested, and a motion in the opposite direction is esta- 

 blished. 



The results thus described exhibit themselves vnth. a machine 

 provided with one rubber at the same height as the axis round 

 which the vertical disc turns. They appear with most regularity 

 when the conductor is removed, but vanish altogether if the 

 nibber also be taken away. The expei'imeut succeeds better 

 when the needle is isolated. With other arrangements of the 

 machine divergent results are obtained, which, however, are easily 

 explained by reference to the origin of the phsenomena. 



We have here to do with the electric attraction of the disc 

 alone. This attraction is not greatest in the direction of the 

 perpendicular from the centre of the needle upon the disc, but 

 in an oblique direction,which points towards the advancing por- 

 tions of the disc. These portions have quitted the rubber later, 

 and therefore possess a more lively electricity than the parts im- 

 mediately opposite to the end of the needle. Hence during the 

 revolution of the disc we have an electric pull directed obliquely 

 against the surface, which becomes stronger us wc a])proach the 

 * Translated from PoggeudorfF's Annalen, vol. Ixxxi. p. 315. 



