of Mahomet's Coffin. 191 



all the lines of force must ultimately find their way to the outer 

 coating or some other neutral position, those which begin by 

 curving upwards must after a time incline in the opposite direc- 

 tion ; and this will take place nearer to the knob the shorter the 

 stem is which raises it. But if the knob be not exorbitantly 

 large, and if it be sufficiently stilted up above the jar, the vertical 

 component of the electrical forces acting on the strip will go on 

 increasing from the top of the ball and for some distance down 

 its sides ; so that if the gold-leaf be not too heavy, it will find a 

 position of stable equilibrium. 



Again, if the resultant of the electrical forces be not directed 

 through the centre of gravity of the gold-leaf, a couple will be 

 formed which will incline the strip of gold-leaf so that, instead of 

 lying along one of the lines of force, it will lie in some degree 

 athwart them. But the new directions of the electrical forces 

 will manifestly produce an opposing couple which will bring about 

 equilibrium. 



And, again, if the electrical forces produce a horizontal as well 

 as a vertical component, this will cause the slip of gold-leaf 

 somewhat to approach or withdraw from the knob. In either 

 case its position may, and probably will, be different from that 

 in which it discharges positive and negative electricity from its 

 two ends in equal quantities, and in which alone its own electrical 

 condition can remain constant. If, then, when the strip is in 

 this position of constant charge, it happen that all the forces 

 acting on it produce e. g. an inward resultant, it must approach 

 the ball and thus come into a position in which it quickly be- 

 comes positively charged, from which it is therefore again driven 

 outwards beyond the position of constant charge into a situation 

 in which it becomes negative, from which it is therefore drawn 

 in again. It thus flutters between two positions on either side 

 of the position of constant charge. I have not seen this motion 

 in the main body of the strip, although it certainly exists; but 

 I have seen a fragment which happened to be slenderly attached 

 to the side of a strip quivering with a motion which was no doubt 

 due to it. 



It now only remains to notice some minor details. I have 

 seen the strip spin on its longer axis. This was doubtless 

 because the jags, which it is difficult to avoid in cutting gold- 

 leaf, happened on that occasion to turn upwards along one side 

 of the strip and downwards along the other, so that it was moved 

 like the electrical fly-wheel. Similarly, if the jags along one side 

 allow a freer escape of electricity than those along the other, as 

 often happens, the little strip traverses horizontally in a cone 

 round the vertical line through the knob; for the little strip of 

 gold commonly assumes a position in which its transverse dia- 



